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/net.rs

@@ -19,7 +19,7 @@ use num::{SignedInt, Int};
 use rt;
 use sync::{Once, ONCE_INIT};
 use sys::c;
-use sys_common::AsInner;
+use sys_common::{AsInner, FromInner};
 
 pub type wrlen_t = i32;
 
@@ -123,10 +123,14 @@ impl Socket {
 
 impl Drop for Socket {
     fn drop(&mut self) {
-        unsafe { cvt(libc::closesocket(self.0)).unwrap(); }
+        let _ = unsafe { libc::closesocket(self.0) };
     }
 }
 
 impl AsInner<libc::SOCKET> for Socket {
     fn as_inner(&self) -> &libc::SOCKET { &self.0 }
 }
+
+impl FromInner<libc::SOCKET> for Socket {
+    fn from_inner(sock: libc::SOCKET) -> Socket { Socket(sock) }
+}