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.
It's a notation for `IsTrunc (-1)`, and according to `Set Printing Universes`, `IsTrunc` only takes one universe argument.