stabilize more of collections

This stabilizes most of libcollections, carefully avoiding sections of API which are being managed in other PRs. APIs which are not stable are marked explicitly unstable with a reason. Also deprecates the following:

* DList
  * rotate_forward
  * rotate_backward
  * prepend
  * insert_when
  * insert_ordered
  * merge

* VecMap
  * update
  * update_with_key

as such this is a

[breaking-change]
cc @aturon, perhaps we should leave these as unstable until `Borrow` it stabilized? Not that it would change the signature much...