Path & PathBuf missing FromStr trait

[mqudsi]

Both std::path::Path and std::path::PathBuf implement std::convert::From<String> but neither one implements the std::str::FromStr trait. Since both Path and PathBuf have infallible conversions from string types, they should implement std::str::FromStr (esp. so that they can be used as generic "convertible from str" types).

Note that if something is done about rust-lang/rfcs#2143, this would be a moot discussion.

[kennytm]

Note that, as discovered from #48292, it is impossible to implement FromStr for Path (requires unsized return value) nor &'a Path (requires GATs).

[SimonSapin]

@rust-lang/libs, do we want to revert this addition #48292 until we can add it back again with ! for the error type? #49039, #50121