Refactor how keywords are matched.

Refactor how keywords are matched.

* spar-arg:compare renamed to spar-arg:id=? and second argument is optional,
  defaulting to spar-arg:form.

* patterns (noise ...), (noise-keyword ...), and (keyword ...) are eliminated.

* New pattern (ignore-if predicate arg...) ignores a subform if calling the
  predicate is satisfied.

* Renamed (value-if ...) to (keep-if ...) with same semantics: keeps a subform
  if calling the predicate is satisfied.

* New pattern (value arg) is like (values arg...) but a little clearer.