Changes to define-generics: added #:defined-predicate; improved generated code.

The #:defined-predicate option is like #:defined-table, but it defines a
two-argument predicate that only processes the requested method name and doesn't
allocate a hash table each time it is called.

Method implementations provided via #:fallbacks, #:defaults, and #:fast-defaults
are now called directly by method procedures rather than stored in a vector and
extracted by index.  This should hopefully improve inlining for methods.
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.
Extended the gen:dict interface; also added some dict and hash operations.

- Added hash-empty?, hash-clear, and hash-clear! for hash tables.
- Added dict-empty?, dict-clear, and dict-clear! for dictionaries.
- Made all dict functions exported by racket/dict into generic methods; turned
  the existing implementations into fallbacks.
Added define-custom-hash-types and make-custom-hash-types.

The definition form and function consume an equality predicate and optional hash
code functions and key predicate, and produce predicates and constructor
functions for hash table-based dictionaries using the given equality and hash
functions.  Immutable, mutable, and weak dictionaries are defined.
Added dict-copy operation.
Implemented gen:set.
Added tests for new kinds of sets.