auto merge of #20356 : Gankro/rust/stab, r=aturon

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.

Deprecates:

* DList
  * rotate_forward
  * rotate_backward
  * prepend
  * insert_when
  * insert_ordered
  * merge

* VecMap
  * update
  * update_with_key

* Renames and newtypes the Bitv and BitvSet iterators to match conventions.

* Removes the Copy impl from DList's Iter.

as such this is a

[breaking-change]

diff --cc src/libcollections/dlist.rs
Simple merge