libcore: DST-ify AsSlice

This commit changes `AsSlice` to work on unsized types, and changes the
`impl` for `&[T]` to `[T]`. Aside from making the trait more general,
this also helps some ongoing work with method resolution changes.

This is a breaking change: code that uses generics bounded by `AsSlice`
will have to change. In particular, such code previously often took
arguments of type `V` where `V: AsSlice<T>` by value. These should now
be taken by reference:

```rust
fn foo<Sized? V: AsSlice<T>>(v: &V) { .. }
```

A few std lib functions have been changed accordingly.

[breaking-change]

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libstd/path/posix.rs b/src/libstd/path/posix.rs
index 3e013ba20c4..2b444fdc32b 100644
--- a/src/libstd/path/posix.rs
+++ b/src/libstd/path/posix.rs
@@ -16,6 +16,7 @@ use cmp::{PartialEq, Eq, PartialOrd, Ord, Ordering};
 use hash;
 use io::Writer;
 use iter::{DoubleEndedIterator, AdditiveIterator, Extend, Iterator, Map};
+use kinds::Sized;
 use option::{Option, None, Some};
 use str::{FromStr, Str};
 use str;
@@ -342,7 +343,7 @@ impl Path {
 
     /// Returns a normalized byte vector representation of a path, by removing all empty
     /// components, and unnecessary . and .. components.
-    fn normalize<V: AsSlice<u8>>(v: V) -> Vec<u8> {
+    fn normalize<Sized? V: AsSlice<u8>>(v: &V) -> Vec<u8> {
         // borrowck is being very picky
         let val = {
             let is_abs = !v.as_slice().is_empty() && v.as_slice()[0] == SEP_BYTE;