RFC: Statically enforce Unicode in std::fmt

Statically enforce that the `std::fmt` module can only create valid UTF-8 data
by removing the arbitrary `write` method in favor of a `write_str` method.
Merge branch 'fmt-text-writer' of https://github.com/alexcrichton/rfcs into alexcrichton-fmt-text-writer
RFC 526 is Statically enforce Unicode in std::fmt
Merge branch 'alexcrichton-fmt-text-writer' into HEAD