Remark: The kernel was already using Sort. So, the limitation was
artificial. Moreover, it may seem unnecessary to have quotients of
proofs in a proof irrelevant system, but this is useful for proving
a more general funext lemma. This more general version is needed in
the new tested contributed by @digama0.
Before this commit, an user could define their own prelude and change
the types of quot, quot.mk, quot.lift or quot.ind.
By doing that, they could prove false.
This commit prevents this kind of abuse.
It also modifies the definition of `quot` and avoids the `setoid`
dependency.
The previous `quot` type is now called `quotient`, and it is defined
using the new `quot` type provided by the kernel.
See discussion at #1330
