Many relatively small changes.

Most interestingly:
* Use [lia] instead of [omega] everywhere
* More many generic lemmas on the memory to the theory on finite maps.
* Many additional list lemmas.
* A new interface for a monad for collections, which is now also used by the
  collection tactics.
* Provide an additional finite collection implementation using unordered lists
  without duplicates removed. This implementation forms a monad (just the list
  monad in disguise).