Auto merge of #8579 - yoav-lavi:squashed-master, r=flip1995

`unnecessary_join` lint

changelog: Adds a lint called ``[`unnecessary_join`]`` that detects cases of `.collect::<Vec<String>>.join("")` or `.collect::<Vec<_>>.join("")` on an iterator, suggesting `.collect::<String>()` instead

Fixes: https://github.com/rust-lang/rust-clippy/issues/8570
This is a reopen of https://github.com/rust-lang/rust-clippy/pull/8573

changelog: add lint [`unnecessary_join`]