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]
I marked the types unstable to indicate they need to be renamed. It seemed fine to stabilize the methods since we consider the rename a breaking change.

I can just do the renaming in this PR if desired, though.