Separated prop:dict from gen:dict.

This work is in preparation for widening the interface of gen:dict with
operations like dict-for-each, dict-update, etc., each with a fallback
implementation.  The property prop:dict, with its documented, fixed-length
vector representation, cannot be extended, whereas a generic with optional
methods can be.
Added #:fast-defaults option to define-generics.

For datatypes that are disjoint with structs that might implement the generic,
this adds a "fast path" implementation that doesn't need to test for struct
properties.
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.