define embedding, (split) surjection, retraction, existential quantifier, 'or' connective
also add a whole bunch of theorems about these definitions
still has two sorry's which can be solved after #564 is closed
examples:
foo_eq : Pi {A B : foo}, _ -> A = B
foo_mk_eq : Pi _, foo.mk _ = foo.mk _ (if constructor is called "bar", then this becomes "bar_eq")
foo_eq_equiv : Pi {A B : foo}, (A = B) ≃ _
also changed priority of some instances of is_trunc
