Optimize `io::Write::write_fmt` for constant strings
When the formatting args to `fmt::Write::write_fmt` are a statically known string, it simplifies to only calling `write_str` without a runtime branch. Do the same in `io::Write::write_fmt` with `write_all`.
Also, match the convention of `fmt::Write` for the name of `args`.
Matthias Krüger
809378bd2e