Refactor Fin(Dom)Functions to use Columns

[epatters]

Since #681, we have two parallel implementations of functions with indexing: the one in FinSets and the columns comprising acsets. We should refactor a Fin(Dom)Function to be a column together with domain and codomains (at least in the case where the function is given by explicit data, as opposed to a callable).

As part of this, we should fix #687.

[kris-brown]

Another issue we've seen in the tests is that the return type of preimage seems to always be a vector, even though there is a type parameter for what type is used internally for storing preimages (e.g. SortedSet or Set). Is this intended or a bug?