std: Add a new `env` module

This is an implementation of [RFC 578][rfc] which adds a new `std::env` module
to replace most of the functionality in the current `std::os` module. More
details can be found in the RFC itself, but as a summary the following methods
have all been deprecated:

[rfc]: https://github.com/rust-lang/rfcs/pull/578

* `os::args_as_bytes`   => `env::args`
* `os::args`            => `env::args`
* `os::consts`          => `env::consts`
* `os::dll_filename`    => no replacement, use `env::consts` directly
* `os::page_size`       => `env::page_size`
* `os::make_absolute`   => use `env::current_dir` + `join` instead
* `os::getcwd`          => `env::current_dir`
* `os::change_dir`      => `env::set_current_dir`
* `os::homedir`         => `env::home_dir`
* `os::tmpdir`          => `env::temp_dir`
* `os::join_paths`      => `env::join_paths`
* `os::split_paths`     => `env::split_paths`
* `os::self_exe_name`   => `env::current_exe`
* `os::self_exe_path`   => use `env::current_exe` + `pop`
* `os::set_exit_status` => `env::set_exit_status`
* `os::get_exit_status` => `env::get_exit_status`
* `os::env`             => `env::vars`
* `os::env_as_bytes`    => `env::vars`
* `os::getenv`          => `env::var` or `env::var_string`
* `os::getenv_as_bytes` => `env::var`
* `os::setenv`          => `env::set_var`
* `os::unsetenv`        => `env::remove_var`

Many function signatures have also been tweaked for various purposes, but the
main changes were:

* `Vec`-returning APIs now all return iterators instead
* All APIs are now centered around `OsString` instead of `Vec<u8>` or `String`.
  There is currently on convenience API, `env::var_string`, which can be used to
  get the value of an environment variable as a unicode `String`.

All old APIs are `#[deprecated]` in-place and will remain for some time to allow
for migrations. The semantics of the APIs have been tweaked slightly with regard
to dealing with invalid unicode (panic instead of replacement).

The new `std::env` module is all contained within the `env` feature, so crates
must add the following to access the new APIs:

    #![feature(env)]

[breaking-change]

3 files changed, 9 insertions, 8 deletions

diff --git a/src/libstd/rt/backtrace.rs b/src/libstd/rt/backtrace.rs
index f2d66e1a4d7..905cc06c4f0 100644
--- a/src/libstd/rt/backtrace.rs
+++ b/src/libstd/rt/backtrace.rs
@@ -14,7 +14,7 @@
 
 use prelude::v1::*;
 
-use os;
+use env;
 use sync::atomic::{self, Ordering};
 
 pub use sys::backtrace::write;
@@ -29,7 +29,7 @@ pub fn log_enabled() -> bool {
         _ => {}
     }
 
-    let val = match os::getenv("RUST_BACKTRACE") {
+    let val = match env::var("RUST_BACKTRACE") {
         Some(..) => 2,
         None => 1,
     };
diff --git a/src/libstd/rt/mod.rs b/src/libstd/rt/mod.rs
index 7e19f1cac2c..00088d6d99a 100644
--- a/src/libstd/rt/mod.rs
+++ b/src/libstd/rt/mod.rs
@@ -65,7 +65,7 @@ fn lang_start(main: *const u8, argc: int, argv: *const *const u8) -> int {
     use prelude::v1::*;
 
     use mem;
-    use os;
+    use env;
     use rt;
     use sys_common::thread_info::{self, NewThread};
     use sys_common;
@@ -131,7 +131,7 @@ fn lang_start(main: *const u8, argc: int, argv: *const *const u8) -> int {
     if failed {
         rt::DEFAULT_ERROR_CODE
     } else {
-        os::get_exit_status()
+        env::get_exit_status() as isize
     }
 }
 
diff --git a/src/libstd/rt/util.rs b/src/libstd/rt/util.rs
index f5727a38b69..86d21cf7278 100644
--- a/src/libstd/rt/util.rs
+++ b/src/libstd/rt/util.rs
@@ -13,6 +13,7 @@
 use prelude::v1::*;
 
 use cmp;
+use env;
 use fmt;
 use intrinsics;
 use libc::{self, uintptr_t};
@@ -51,7 +52,7 @@ pub fn min_stack() -> uint {
         0 => {}
         n => return n - 1,
     }
-    let amt = os::getenv("RUST_MIN_STACK").and_then(|s| s.parse().ok());
+    let amt = env::var_string("RUST_MIN_STACK").ok().and_then(|s| s.parse().ok());
     let amt = amt.unwrap_or(2 * 1024 * 1024);
     // 0 is our sentinel value, so ensure that we'll never see 0 after
     // initialization has run
@@ -62,15 +63,15 @@ pub fn min_stack() -> uint {
 /// Get's the number of scheduler threads requested by the environment
 /// either `RUST_THREADS` or `num_cpus`.
 pub fn default_sched_threads() -> uint {
-    match os::getenv("RUST_THREADS") {
-        Some(nstr) => {
+    match env::var_string("RUST_THREADS") {
+        Ok(nstr) => {
             let opt_n: Option<uint> = nstr.parse().ok();
             match opt_n {
                 Some(n) if n > 0 => n,
                 _ => panic!("`RUST_THREADS` is `{}`, should be a positive integer", nstr)
             }
         }
-        None => {
+        Err(..) => {
             if limit_thread_creation_due_to_osx_and_valgrind() {
                 1
             } else {