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
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
The new command provides a uniform way to set declaration attributes.
It replaces the commands: class, instance, coercion, multiple_instances,
reducible, irreducible
After this commit, Lean "cuts" the search after the first instance is
computed. To obtain the previous behavior, we must use the new command
multiple_instances <class-name>
closes#370
