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/fs2.rs

@@ -280,6 +280,12 @@ fn cstr(path: &Path) -> io::Result<CString> {
     Ok(cstring)
 }
 
+impl FromInner<c_int> for File {
+    fn from_inner(fd: c_int) -> File {
+        File(FileDesc::new(fd))
+    }
+}
+
 pub fn mkdir(p: &Path) -> io::Result<()> {
     let p = try!(cstr(p));
     try!(cvt(unsafe { libc::mkdir(p.as_ptr(), 0o777) }));