std: Stabilize the std::hash module

This commit aims to prepare the `std::hash` module for alpha by formalizing its
current interface whileholding off on adding `#[stable]` to the new APIs.  The
current usage with the `HashMap` and `HashSet` types is also reconciled by
separating out composable parts of the design. The primary goal of this slight
redesign is to separate the concepts of a hasher's state from a hashing
algorithm itself.

The primary change of this commit is to separate the `Hasher` trait into a
`Hasher` and a `HashState` trait. Conceptually the old `Hasher` trait was
actually just a factory for various states, but hashing had very little control
over how these states were used. Additionally the old `Hasher` trait was
actually fairly unrelated to hashing.

This commit redesigns the existing `Hasher` trait to match what the notion of a
`Hasher` normally implies with the following definition:

    trait Hasher {
        type Output;
        fn reset(&mut self);
        fn finish(&self) -> Output;
    }

This `Hasher` trait emphasizes that hashing algorithms may produce outputs other
than a `u64`, so the output type is made generic. Other than that, however, very
little is assumed about a particular hasher. It is left up to implementors to
provide specific methods or trait implementations to feed data into a hasher.

The corresponding `Hash` trait becomes:

    trait Hash<H: Hasher> {
        fn hash(&self, &mut H);
    }

The old default of `SipState` was removed from this trait as it's not something
that we're willing to stabilize until the end of time, but the type parameter is
always required to implement `Hasher`. Note that the type parameter `H` remains
on the trait to enable multidispatch for specialization of hashing for
particular hashers.

Note that `Writer` is not mentioned in either of `Hash` or `Hasher`, it is
simply used as part `derive` and the implementations for all primitive types.

With these definitions, the old `Hasher` trait is realized as a new `HashState`
trait in the `collections::hash_state` module as an unstable addition for
now. The current definition looks like:

    trait HashState {
        type Hasher: Hasher;
        fn hasher(&self) -> Hasher;
    }

The purpose of this trait is to emphasize that the one piece of functionality
for implementors is that new instances of `Hasher` can be created.  This
conceptually represents the two keys from which more instances of a
`SipHasher` can be created, and a `HashState` is what's stored in a
`HashMap`, not a `Hasher`.

Implementors of custom hash algorithms should implement the `Hasher` trait, and
only hash algorithms intended for use in hash maps need to implement or worry
about the `HashState` trait.

The entire module and `HashState` infrastructure remains `#[unstable]` due to it
being recently redesigned, but some other stability decision made for the
`std::hash` module are:

* The `Writer` trait remains `#[experimental]` as it's intended to be replaced
  with an `io::Writer` (more details soon).
* The top-level `hash` function is `#[unstable]` as it is intended to be generic
  over the hashing algorithm instead of hardwired to `SipHasher`
* The inner `sip` module is now private as its one export, `SipHasher` is
  reexported in the `hash` module.

And finally, a few changes were made to the default parameters on `HashMap`.

* The `RandomSipHasher` default type parameter was renamed to `RandomState`.
  This renaming emphasizes that it is not a hasher, but rather just state to
  generate hashers. It also moves away from the name "sip" as it may not always
  be implemented as `SipHasher`. This type lives in the
  `std::collections::hash_map` module as `#[unstable]`

* The associated `Hasher` type of `RandomState` is creatively called...
  `Hasher`! This concrete structure lives next to `RandomState` as an
  implemenation of the "default hashing algorithm" used for a `HashMap`. Under
  the hood this is currently implemented as `SipHasher`, but it draws an
  explicit interface for now and allows us to modify the implementation over
  time if necessary.

There are many breaking changes outlined above, and as a result this commit is
a:

[breaking-change]

2 files changed, 29 insertions, 32 deletions

diff --git a/src/libserialize/collection_impls.rs b/src/libserialize/collection_impls.rs
index d89a4754d2e..42498328ff6 100644
--- a/src/libserialize/collection_impls.rs
+++ b/src/libserialize/collection_impls.rs
@@ -13,6 +13,7 @@
 use std::uint;
 use std::default::Default;
 use std::hash::{Hash, Hasher};
+use std::collections::hash_state::HashState;
 
 use {Decodable, Encodable, Decoder, Encoder};
 use std::collections::{DList, RingBuf, BTreeMap, BTreeSet, HashMap, HashSet, VecMap};
@@ -156,13 +157,12 @@ impl<
     }
 }
 
-#[old_impl_check]
-impl<
-    K: Encodable + Hash<X> + Eq,
-    V: Encodable,
-    X,
-    H: Hasher<X>
-> Encodable for HashMap<K, V, H> {
+impl<K, V, S> Encodable for HashMap<K, V, S>
+    where K: Encodable + Hash< <S as HashState>::Hasher> + Eq,
+          V: Encodable,
+          S: HashState,
+          <S as HashState>::Hasher: Hasher<Output=u64>
+{
     fn encode<S: Encoder>(&self, e: &mut S) -> Result<(), S::Error> {
         e.emit_map(self.len(), |e| {
             let mut i = 0;
@@ -176,17 +176,16 @@ impl<
     }
 }
 
-#[old_impl_check]
-impl<
-    K: Decodable + Hash<S> + Eq,
-    V: Decodable,
-    S,
-    H: Hasher<S> + Default
-> Decodable for HashMap<K, V, H> {
-    fn decode<D: Decoder>(d: &mut D) -> Result<HashMap<K, V, H>, D::Error> {
+impl<K, V, S> Decodable for HashMap<K, V, S>
+    where K: Decodable + Hash< <S as HashState>::Hasher> + Eq,
+          V: Decodable,
+          S: HashState + Default,
+          <S as HashState>::Hasher: Hasher<Output=u64>
+{
+    fn decode<D: Decoder>(d: &mut D) -> Result<HashMap<K, V, S>, D::Error> {
         d.read_map(|d, len| {
-            let hasher = Default::default();
-            let mut map = HashMap::with_capacity_and_hasher(len, hasher);
+            let state = Default::default();
+            let mut map = HashMap::with_capacity_and_hash_state(len, state);
             for i in range(0u, len) {
                 let key = try!(d.read_map_elt_key(i, |d| Decodable::decode(d)));
                 let val = try!(d.read_map_elt_val(i, |d| Decodable::decode(d)));
@@ -197,12 +196,11 @@ impl<
     }
 }
 
-#[old_impl_check]
-impl<
-    T: Encodable + Hash<X> + Eq,
-    X,
-    H: Hasher<X>
-> Encodable for HashSet<T, H> {
+impl<T, S> Encodable for HashSet<T, S>
+    where T: Encodable + Hash< <S as HashState>::Hasher> + Eq,
+          S: HashState,
+          <S as HashState>::Hasher: Hasher<Output=u64>
+{
     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_seq(self.len(), |s| {
             let mut i = 0;
@@ -215,15 +213,15 @@ impl<
     }
 }
 
-#[old_impl_check]
-impl<
-    T: Decodable + Hash<S> + Eq,
-    S,
-    H: Hasher<S> + Default
-> Decodable for HashSet<T, H> {
-    fn decode<D: Decoder>(d: &mut D) -> Result<HashSet<T, H>, D::Error> {
+impl<T, S> Decodable for HashSet<T, S>
+    where T: Decodable + Hash< <S as HashState>::Hasher> + Eq,
+          S: HashState + Default,
+          <S as HashState>::Hasher: Hasher<Output=u64>
+{
+    fn decode<D: Decoder>(d: &mut D) -> Result<HashSet<T, S>, D::Error> {
         d.read_seq(|d, len| {
-            let mut set = HashSet::with_capacity_and_hasher(len, Default::default());
+            let state = Default::default();
+            let mut set = HashSet::with_capacity_and_hash_state(len, state);
             for i in range(0u, len) {
                 set.insert(try!(d.read_seq_elt(i, |d| Decodable::decode(d))));
             }
diff --git a/src/libserialize/lib.rs b/src/libserialize/lib.rs
index 139170fc012..fd155261934 100644
--- a/src/libserialize/lib.rs
+++ b/src/libserialize/lib.rs
@@ -24,7 +24,6 @@ Core encoding and decoding interfaces.
        html_playground_url = "http://play.rust-lang.org/")]
 #![allow(unknown_features)]
 #![feature(slicing_syntax)]
-#![feature(old_impl_check)]
 #![cfg_attr(stage0, allow(unused_attributes))]
 
 // test harness access