Added a #:derive-property option to define-generics.

The define-generics form can now derive existing struct properties, so that any
instance of the new generics group is also an instance of the struct property.
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.
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; renamed dict-supports(?|/c) to dict-implements(?|/c).

Similar to hash-copy or string-copy, dict-copy creates a new, mutable copy of
the given dictionary.  Added as a method of gen:dict.
Added gen:set, mutable and weak sets, and custom set types.

Sets are now implemented as a generic interface, and lists count as sets.  Most
of the set functions have been added as methods, including mutable versions of
imperative update methods.
Added tests for mutable, weak, and custom sets.
Documented new options for define-generics.

Added documentation for #:fast-defaults, #:fallbacks, and #:defined-predicate.
Also documented #:defined-table as deprecated in favor of #:defined-predicate,
and touched up other parts of the documentation, mostly by reordering.
Documented changes to racket/dict.

Reorganized the documentation with a separate subsection for gen:dict methods,
split into primitive and derived depending on whether they have fallback
implementations.  Added the new methods dict-empty?, dict-copy, dict-clear, and
dict-clear!.  Documented dict-supports?, dict-support/c, make-custom-hash-types,
and define-custom-hash-types.
Documented changes to racket/set.

Added documentation gen:set and reorganized documentation to set aside functions
that are methods of gen:set.  Documented new mutable, weak, and custom set
types.  Documented new methods for mutating sets.
Changed "generics group" to "generic interface" in errors and documentation.
Added tools for contracts, impersonators, and chaperones for generic interfaces.

Added four macros to racket/generic:
- chaperone-generics
- impersonate-generics
- redirect-generics (dynamically chooses between the above two)
- generic-instance/c
All take pairs of method names and wrappers (procedures or contracts),
and turn those into appropriate chaperone, impersonator, or contract wrappers
on the method tables of the given structs.

Used this to rewrite set/c to give better error messages.
Documented forms for generic instance chaperones, impersonators, and contracts.