remove funext class,
remove a couple of sorry's,
add characterization of equality in trunctypes,
use Jeremy's format for headers everywhere in the HoTT library,
continue working on Yoneda embedding
The convention is this: we use e.g. nat.is_inhabited and nat.has_decidable_eq
for these two purposes only, to avoid clashing with "inhabited" and "decidable_eq"
in a namespace. Otherwise, we use "decidable_foo" and "inhabited_foo".
also add more definitions in types.pi, types.path, algebra.precategory
the (pre)category library still needs cleanup
authors of this commit: @avigad, @javra, @fpvandoorn
