std: Stabilize the ffi module

The two main sub-modules, `c_str` and `os_str`, have now had some time to bake
in the standard library. This commits performs a sweep over the modules adding
various stability tags.

The following APIs are now marked `#[stable]`

* `OsString`
* `OsStr`
* `OsString::from_string`
* `OsString::from_str`
* `OsString::new`
* `OsString::into_string`
* `OsString::push` (renamed from `push_os_str`, added an `AsOsStr` bound)
* various trait implementations for `OsString`
* `OsStr::from_str`
* `OsStr::to_str`
* `OsStr::to_string_lossy`
* `OsStr::to_os_string`
* various trait implementations for `OsStr`
* `CString`
* `CStr`
* `NulError`
* `CString::new` - this API's implementation may change as a result of
  rust-lang/rfcs#912 but the usage of `CString::new(thing)` looks like it is
  unlikely to change. Additionally, the `IntoBytes` bound is also likely to
  change but the set of implementors for the trait will not change (despite the
  trait perhaps being renamed).
* `CString::from_vec_unchecked`
* `CString::as_bytes`
* `CString::as_bytes_with_nul`
* `NulError::nul_position`
* `NulError::into_vec`
* `CStr::from_ptr`
* `CStr::as_ptr`
* `CStr::to_bytes`
* `CStr::to_bytes_with_nul`
* various trait implementations for `CStr`

The following APIs remain `#[unstable]`

* `OsStr*Ext` traits remain unstable as the organization of `os::platform` is
  uncertain still and the traits may change location.
* `AsOsStr` remains unstable as generic conversion traits are likely to be
  rethought soon.

The following APIs were deprecated

* `OsString::push_os_str` is now called `push` and takes `T: AsOsStr` instead (a
  superset of the previous functionality).

src/librustdoc/html/render.rs

@@ -38,7 +38,6 @@ use std::cell::RefCell;
 use std::cmp::Ordering;
 use std::collections::{HashMap, HashSet};
 use std::default::Default;
-use std::ffi::OsStr;
 use std::fmt;
 use std::fs::{self, File};
 use std::io::prelude::*;
@@ -770,7 +769,7 @@ impl<'a> SourceCollector<'a> {
 
         let mut fname = p.file_name().expect("source has no filename")
                          .to_os_string();
-        fname.push_os_str(OsStr::from_str(".html"));
+        fname.push(".html");
         cur.push(&fname);
         let mut w = BufWriter::new(try!(File::create(&cur)));