std: Fixup some missing stabilization on str

std: Fixup some missing stabilization on str

* The `str` module itself is stable.
* The `StrExt` trait is stable (and impls).
* The `Utf8Error` type is unstable.
* The `from_utf8` function is stable
* Some iterators are now stable:
  * `Chars`
  * `CharIndices`
* The `MatchIndices` iterator is now unstable
* The public `traits` module is no longer public.