Stabilize std::borrow

This commit stabilizes `std::borrow`, making the following modifications
to catch up the API with language changes:

* It renames `BorrowFrom` to `Borrow`, as was originally intended (but
  blocked for technical reasons), and reorders the parameters
  accordingly.

* It moves the type parameter of `ToOwned` to an associated type. This
  is somewhat less flexible, in that each borrowed type must have a
  unique owned type, but leads to a significant simplification for
  `Cow`. Flexibility can be regained by using newtyped slices, which is
  advisable for other reasons anyway.

* It removes the owned type parameter from `Cow`, making the type much
  less verbose.

* Deprecates the `is_owned` and `is_borrowed` predicates in favor of
  direct matching.

The above API changes are relatively minor; the basic functionality
remains the same, and essentially the whole module is now marked
`#[stable]`.

[breaking-change]

0 files changed, 0 insertions, 0 deletions