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.