CodeFormatter should use `T: ?Sized + fmt::Write` instead of `T: fmt::Write`

[thomcc]

That way CodeFormatter<'a, dyn fmt::Write> is usable. This is trivial (bounds just need to gain : ?Sized), non-breaking, and has no real downsides AFAICT.

The only reason this is an issue and not a PR is that it would have a merge conflict with #15, so I'll wait for that one to resolve either way first rather than having a mid-air collision with myself.