mathlib3

fc57cf49 - feat(data/{finset,finsupp,multiset}): more assorted lemmas (#4006)

feat(data/{finset,finsupp,multiset}): more assorted lemmas (#4006) Another grab bag of facts from the Witt vector branch. Coauthored by: Johan Commelin <johan@commelin.net> <!-- put comments you want to keep out of the PR commit here --> Co-authored-by: Johan Commelin <johan@commelin.net>