Use static dispatch for Formatter methods in Rust code

[Evian-Zhang]

Currently, methods of Formatter accept a &mut dyn FormatterOutput for writing formatted output. Why don't we use &mut impl FormatterOutput, which could leverage the benefits of inlining brought by static dispatch? Is this decision because of MSRV?

I tried to text-replace all &mut dyn FormatterOutput to &mut impl FormatterOutput, and things work fine in the latest Rust stable compiler, only the tests cannot compile since we cannot make Formatter into a trait object any more. But this can be fixed by adjusting test code (and I can't figure out any usecases where downstream users would use Box<dyn Formatter>).

If this change is acceptable, I would further fix the code in unit test to make it pass the tests, and file a PR afterwards. :)