This adds the requirement that all function implementations provider a
String() string method so that these can be used as vertices in the
pgraph library. If we eventually move to generics for the pgraph DAG,
then this might not matter, but it's not bad that these have names
either.
This flattens the type unification of the map function so that the
solver has more to work with. It's possible that some scenarios might
solve faster, or without recursion, after this improvement.
