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.

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[clarfonthey]

@rustbot author

[rustbot]

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

[clarfonthey]

Now that I know the workaround for the ./x test library/std, this should be good. But I'll wait until the tests pass (I'm only running on Linux atm) before I mark as ready.

There are a few edge cases, namely removing the trailing separator from /.

[clarfonthey]

@rustbot ready

[bors]

☔ The latest upstream changes (presumably #145334) made this pull request unmergeable. Please resolve the merge conflicts.

[joboet]

Is this correct? I would have thought that since // is the same as /, this would trim // to / (but feel free to correct me).

[clarfonthey]

The idea here is that removing any number of /s would not affect the final path, and so, the path is left as-is. Essentially, any sequence of separators is treated as a single separator for the logic.

I could change it so that it returns / here but it felt better to be conservative: the logic is that trailing separators are removed if doing so would result in the same path without a trailing separator. Since there will always be trailing separators here, it doesn't make a difference to remove any. If your goal is "cleaning up" the path, then normalize and normalize_lexically exist instead.