collections: Move push/pop to MutableSeq

Implement for Vec, DList, RingBuf. Add MutableSeq to the prelude.

Since the collections traits are in the prelude most consumers of
these methods will continue to work without change.

[breaking-change]
collections: Deprecate push_back/pop_back
Convert some push_back users to push
collections: Move push/pop docs to MutableSeq
vim: Add MutableSeq
collections: Make push_back/pop_back default methods
collections: Deprecate shift/unshift

Use insert/remove instead.
Remove kludgy imports from vec! macro
collections: Tweak docs for push
Remove stray llvmdeps.rs