Add Path::has_trailing_sep and related methods

[clarfonthey]

Implements rust-lang/libs-team#335.

Tracking issue: #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.

[rustbot]

r? @joboet

rustbot has assigned @joboet.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[clarfonthey]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rust-log-analyzer]

The job mingw-check-2 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)