Format ranges

[nrc]

We currently only format a whole file. We should also try to format a range. Rustfmt has partial support for this (rust-lang/rustfmt#1159) I think it is good enough, that we should implement in the RLS (as an unstable feature) and then let Rustfmt catch up.

[kogai]

I want to challenge this.
Probably, I start from reading ActionHandler.reformat.

[nrc]

@kogai yes, it should look pretty similar to reformat. In Rustfmt, you'll want to look at the "file-lines" feature which lets you pass the range to Rustfmt.