Added #:dispatch option for clauses in #:defaults and #:fast-defaults.

The #:dispatch option specifies a second, less strict predicate for a #:defaults
or #:fast-defaults clause.  The main predicate is still used for the generics
group's type predicate, but the dispatch predicate is used in choosing which
method implementation to use.  The #:dispatch option is useful when the dispatch
predicate is disjoint from all other instances, is significantly cheaper to run
than the main predicate, and the full checks can be meaningfully deferred in
method implementations.

Specifically, this is useful in the implementation of iteration for
association-list dictionaries.  The dict-iterate-{next,key,value} functions do
not need to test (andmap pair? dict) if the given dict is eq? to the one stored
in the given iterator.  The dispatch predicate, list?, is much cheaper.