rollup merge of #21759: aturon/new-path

rollup merge of #21759: aturon/new-path

This PR implements [path reform](https://github.com/rust-lang/rfcs/pull/474), and motivation and details for the change can be found there.

For convenience, the old path API is being kept as `old_path` for the time being. Updating after this PR is just a matter of changing imports to `old_path` (which is likely not needed, since the prelude entries still export the old path API).

This initial PR does not include additional normalization or platform-specific path extensions. These will be done in follow up commits or PRs.

[breaking-change]

Closes #20034
Closes #12056
Closes #11594
Closes #14028
Closes #14049
Closes #10035