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/unix/net.rs

@@ -17,7 +17,7 @@ use str;
 use sys::c;
 use net::SocketAddr;
 use sys::fd::FileDesc;
-use sys_common::AsInner;
+use sys_common::{AsInner, FromInner};
 
 pub use sys::{cvt, cvt_r};
 
@@ -72,3 +72,7 @@ impl Socket {
 impl AsInner<c_int> for Socket {
     fn as_inner(&self) -> &c_int { self.0.as_inner() }
 }
+
+impl FromInner<c_int> for Socket {
+    fn from_inner(fd: c_int) -> Socket { Socket(FileDesc::new(fd)) }
+}