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/libstd/lib.rs

@@ -122,12 +122,11 @@
 #![feature(unsafe_no_drop_flag, filling_drop)]
 #![feature(macro_reexport)]
 #![feature(unique)]
-#![feature(convert)]
 #![feature(allow_internal_unstable)]
 #![feature(str_char)]
 #![feature(into_cow)]
-#![feature(slice_patterns)]
 #![feature(std_misc)]
+#![feature(slice_patterns)]
 #![feature(debug_builders)]
 #![cfg_attr(test, feature(test, rustc_private, std_misc))]