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.

[Rendered](https://github.com/alexcrichton/rfcs/blob/fmt-text-writer/text/0000-fmt-text-writer.md)
This is [`std::fmt::Result`](http://doc.rust-lang.org/std/fmt/type.Result.html).