A new version of Hurkens.v.

Adds a more generic and modular proof of Hurkens, where a shallow embedding of U- is given in the most general way. Subsumes all the previous proofs, opens the way to new proofs.
Hurkens.v: coqdoc documentation.
Hurkens.v: a proof of [Type@{i}<>A] for any [A:Type@{i}].
Hurkens.v: show proofs in coqdoc.