Add solver `multiset_solver` for multisets
The code contains some documentation how it works.
I added a number of tests, and used it to automate many existing lemmas in the
Note that this required restructuring the file quite a bit, since I needed some basic lemmas (now in section
basic_lemmas) to define the tactic, and wanted to use the tactic subsequently to prove many of the existing lemmas (now in section
multiset_unfold). None of the lemma statements changed, only many proofs are replaced by a mere call to