Well-known closures represented using pairs or vectors

Well-known closures represented using pairs or vectors

* module/language/cps/closure-conversion.scm (convert-free-var):
  (convert-free-vars): Take self-known? param, to do the right thing for
  well-known closures.
  (allocate-closure): New helper.  Well-known closures are represented
  using pairs or vectors.
  (init-closure): Adapt tpo DTRT for well-known closures.
  (prune-free-vars): Move up.
  (convert-one): Adapt to new well-known closure representation.