Started documenting #:fallbacks for generics; examples currently broken.
Abstracting out static information about generics.

This work is *unfinished*.
It will allow #:methods, #:defaults, and #:fallbacks clauses to all work
the same, using define/generic where appropriate and producing a method table.
Got racket/private/generic-methods to compile.
Imported racket/private/generic-methods in racket/private/generic.