Some constraints were being lost with the previous interface.
This is why we had a workaround in fintype.lean.
We can also remove some hacks we have used in the past.
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
