plugins (perf): Add a lint for usages of to_string()

Lint by @seanmonstar

`str::to_string()` goes through a `Formatter`, `str::into_string()` is a direct copy and is apparently 5x faster.

I haven't properly benchmarked this yet, but a couple of runs with `-m` seem promising.
We might not want to merge this until we're sure that it's a net plus.

(I'm okay with keeping this lint on `Warn` instead of `Deny`, since at least at the moment warnings get 
fixed eventually)