std: Stabilize parts of std::os::platform::io

This commit stabilizes the platform-specific `io` modules, specifically around
the traits having to do with the raw representation of each object on each
platform.

Specifically, the following material was stabilized:

* `AsRaw{Fd,Socket,Handle}`
* `RawFd` (renamed from `Fd`)
* `RawHandle` (renamed from `Handle`)
* `RawSocket` (renamed from `Socket`)
* `AsRaw{Fd,Socket,Handle}` implementations
* `std::os::{unix, windows}::io`

The following material was added as `#[unstable]`:

* `FromRaw{Fd,Socket,Handle}`
* Implementations for various primitives

There are a number of future improvements that are possible to make to this
module, but this should cover a good bit of functionality desired from these
modules for now. Some specific future additions may include:

* `IntoRawXXX` traits to consume the raw representation and cancel the
  auto-destructor.
* `Fd`, `Socket`, and `Handle` abstractions that behave like Rust objects and
  have nice methods for various syscalls.

At this time though, these are considered backwards-compatible extensions and
will not be stabilized at this time.

This commit is a breaking change due to the addition of `Raw` in from of the
type aliases in each of the platform-specific modules.

[breaking-change]

src/libstd/sys/windows/os.rs

@@ -31,7 +31,7 @@ use ptr;
 use slice;
 use sys::c;
 use sys::fs::FileDesc;
-use sys::handle::Handle as RawHandle;
+use sys::handle::Handle;
 
 use libc::funcs::extra::kernel32::{
     GetEnvironmentStringsW,
@@ -369,7 +369,7 @@ pub fn home_dir() -> Option<PathBuf> {
         if c::OpenProcessToken(me, c::TOKEN_READ, &mut token) == 0 {
             return None
         }
-        let _handle = RawHandle::new(token);
+        let _handle = Handle::new(token);
         super::fill_utf16_buf_new(|buf, mut sz| {
             match c::GetUserProfileDirectoryW(token, buf, &mut sz) {
                 0 if libc::GetLastError() != 0 => 0,