Apply under binders

This tactic will be useful for writing a rewrite modulo associativity
tactic.
remove unicode in ReflectiveSubuniverse.v
rename subuniverse predicate more descriptively
O_modal_equiv -> O_modal_isequiv
fix up a few proofs
simplify proof of universality_unit
simplify functoriality proofs
give a type to O_rec_sect
further simplify proof of functoriality for composition
simplify proof of universal property for modal product
simplify proof of sigma_subuniverse and rename
in_subuniverse_paths
make O_unit_isequiv_iff_modal an iff rather than an equality
oops
add brackets for coqdoc
Allow [binder apply] to take a side-condition tac

Also, add an example with explanation.
Add a shorter more basic [binder apply] example

Also add [test] and [not] tacticals to [Overture.v].  For explanatory
purposes, there's now a [binder apply lem], not just [binder apply lem
in H].
avoid (is)equiv_ap
Merge pull request #444 from JasonGross/binder-apply

Apply under binders
Merge pull request #448 from mikeshulman/rsu

improve some proofs and names in ReflectiveSubuniverse.v