Simplify the definition of Vect.reverse
Uncomment Vect.mapMaybe

There was a comment on it indicating that it made the type checker
loop. This seems to no longer be the case.
Remove unnecessary tactic proof from Vect.intersperse
Formatting cleanups in Prelude.Vect
Merge pull request #1560 from david-christiansen/pprint-printdef

Minor cleanups and improvements to Prelude.Vect