Interoperation between ACSets, FinSets, and FinCats

We have dedicated, efficient data structures for acsets. It is also the case that a `FinDomFunctor` sending morphisms to `FinDomFunction`s can be regarded as an acset. These two approaches to the same concept should be made interoperable. The ongoing work on data migration (e.g. #532) makes this very relevant, since data migrations naturally return `FinDomFunctor`s but we ultimately want to get back an acset.

One direction is easy: I have already made a `FinDomFunctor` subtype that wraps an acset. Doing the other direction well may require some internal refactoring. @olynch's suggestions in the related issue #489 would actually jive very well with these requirements.