Remove unused function in univ
Cleanup in Univ, moving code for UniverseConstraints outside the kernel in Universes.
Needed to exponse compare_head_gen(_leq) so that it could be reused in Universes.
Remove unused functions from univ as well and refactor a little bit.
Changed the syntax to Type@{} for explicit universe level specs, following the WG decision.
Fix module order in printers.mllib