Stabilize std::convert and related code

* Marks `#[stable]` the contents of the `std::convert` module.

* Added methods `PathBuf::as_path`, `OsString::as_os_str`,
  `String::as_str`, `Vec::{as_slice, as_mut_slice}`.

* Deprecates `OsStr::from_str` in favor of a new, stable, and more
  general `OsStr::new`.

* Adds unstable methods `OsString::from_bytes` and `OsStr::{to_bytes,
  to_cstring}` for ergonomic FFI usage.

[breaking-change]

src/test/run-pass/env-home-dir.rs

@@ -11,7 +11,6 @@
 // pretty-expanded FIXME #23616
 
 #![feature(path)]
-#![feature(convert)]
 
 use std::env::*;
 use std::path::PathBuf;

src/test/run-pass/issue-20797.rs

@@ -12,8 +12,6 @@
 
 // pretty-expanded FIXME #23616
 
-#![feature(convert)]
-
 use std::default::Default;
 use std::io;
 use std::fs;