rollup merge of #21759: aturon/new-path

This PR implements [path reform](rust-lang/rfcs#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

maybe_owned_vec.rs

@@ -17,7 +17,7 @@ use std::cmp::Ordering;
 use std::default::Default;
 use std::fmt;
 use std::iter::FromIterator;
-use std::path::BytesContainer;
+use std::old_path::BytesContainer;
 use std::slice;
 
 // Note 1: It is not clear whether the flexibility of providing both