Add `Path::has_trailing_sep` and related methods
Implements rust-lang/libs-team#335.
Tracking issue: rust-lang/rust#142503
Notable differences from ACP:
* `trim_trailing_sep` was added to `Path` since it felt reasonable to ensure that the inverse operation was available.
* Per suggestion of `@kennytm,` added `push_trailing_sep` and `pop_trailing_sep` to `PathBuf` in addition to `set_trailing_sep`.
This also updates some of the docs on various `Path` methods to use the term "trailing separator" instead of "trailing slash" for consistency.
