Perhaps, we should add an option to disable this new feature.
Remark: this commit makes commit 46d418a redundant.
I'm keeping 46d418a because we may retract this commit in the future.
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
