Implement fmt::Write for CxxString

[parasyte]

This allows formatted writes (e.g. with the write!() macro) directly on a CxxString. This can eliminate a needless allocation from formatting to a String then converting to a CxxString.

While testing this, I ran into #1201. Nothing else stood out as unusual.

[dtolnay]

I am not sure whether std::fmt::Write or std::io::Write is more appropriate. In the Rust standard library, String implements fmt::Write but not io::Write, while Vec<u8> implements io::Write and not fmt::Write. CxxString has similarities to both in different ways.

If we add fmt::Write now then adding io::Write later is a breaking change because it would make cxx_string.write_fmt(…) ambiguous in the case that you have both traits in scope.

Thoughts?

[parasyte]

Great observation! I can add an impl for io::Write on CxxVector<u8> to this PR. That should address the semver hazards. Are there any other concerns that come to mind?

Ah, and I did not consider implementing io::Write on CxxString, but I could see an argument for it.

[parasyte]

@dtolnay I have addressed the concerns with breaking changes by implementing both fmt::Write and io::Write. I decided not to do anything with CxxVector because I don't need it and it's more work to test.

[dtolnay]

Thanks!