feat(algebra/homology/exact): lemmas about exactness (#4106)
These are a few lemmas on the way to showing how `exact` changes under isomorphisms applied to the objects. It's not everthing one might want; I'm salvaging this from an old branch and unlikely to do more in the near future, but hopefully this is mergeable progress as is.
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
