R1:906ac21e543d

Begin construction of `bin/celerity map`

Summary:
Ref T4222. Moves us toward a more modern Celerity CLI, and moves map discovery into the classtree. This is a little bit bulky (and means you can't ship completely standalone celerity maps) but has the advantage of being completely standard, and we could subclass maps into an auto-discovering map later if we have a need for it.

This doesn't affect the existing Celerity stuff. I'm going to build the new stuff in parallel, and then swap us over at the end.