Per [RFC 517](https://github.com/rust-lang/rfcs/pull/575/), this commit
introduces platform-native strings. The API is essentially as described
in the RFC.
The WTF-8 implementation is adapted from @SimonSapin's
[implementation](https://github.com/SimonSapin/rust-wtf8). To make this
work, some encodign and decoding functionality in `libcore` is now
exported in a "raw" fashion reusable for WTF-8. These exports are *not*
reexported in `std`, nor are they stable.
