simplify sectioning and hypotheses in Functorish
cancelL_isequiv should not be an Instance (can't guess f)
track axioms more carefully in ReflectiveSubuniverse
implicit arguments to O_functor
more functoriality of O
use modalities for functoriality in Truncations
reflectors preserve products
Merge pull request #485 from mikeshulman/functor

Functoriality and product-preservation of reflective subuniverses