clippy::unnecessary_join

Hey,

I've contributed a new lint to clippy called unnecessary_join, it detects cases of .collect::<Vec<String>>().join("") which is longer and usually* less performant than .collect::<String>(). Sharing this here for visibility since it's currently in the pedantic category and isn't automatically used.

\* There are specific cases where this results in loops not being unrolled by the compiler which results in worse performance, so benchmarks should be used if performance critical