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]
This and the iterators below should also have the returned type name altered.