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 seems mostly like a rustdoc bug; e.g. rustdoc could/should show all signatures elided by default, with an option to display elided lifetimes, entirely independent of whether or not there are lifetimes in the source.