Rollup merge of #140956 - Kixunil:impl-partialeq-str-for-path, r=Amanieu

`impl PartialEq<{str,String}> for {Path,PathBuf}`

This is a revival of #105877

Comparison of paths and strings is expected to be possible and needed e.g. in tests. This change adds the impls os `PartialEq` between strings and paths, both owned and unsized, in both directions.

ACP: rust-lang/libs-team#151

Author: Zalathar