Factorization systems

Of course, this is preliminary to constructing the connected/truncated factorization system for any modality.

This file is twice as slow to compile as the previously slowest file (namely `V.v`).  Adding explicit universe annotations sped it up by a couple of seconds already.  Any other suggestions would be welcome.
Do you want to pass universes to `IsHProp`, too?  I think it takes at least one universe beyond its argument.