rewrite (and/c pair? (listof ...)) into (non-empty-listof ...)

related to PR 14832
generalize the random contract generation APIs to allow
a more dynamic notion of failure

use that to try harder with and/c contracts. In particular,
the contract system now tries to generate the arguments
on their own and then uses the other arguments to filter
(when they are all flat contracts, of course)

closes PR 14832
fix incorrect fix from 5495595535 for #:generate default arg
Add some TLC
Fixing JS insertion error. Thanks Tony
Cleanup small errors
Moving enumerator
Adding contracts to data/enumerate
Adding documentation
Fixing exports and actually adding docs
syntax/parse: fix action patterns in splicing stxclass
  closes PR 14837
Adding dep that raco setup doesn't show on my machine
Typo, fixed