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.
I think it would be a bit nicer to put `Build_PathFactorization` on the outside, and do the path induction separately in each component.  This allows projections on factorizations to judgmentally reduce