Added a #:fallbacks option to define-generics.

The #:fallbacks option specifies a set of method definitions.  Each definition
is used for any value that does not support the specific method.  Like
allows define/generic and absent method definitions.  If neither a specific
implementation or the #:fallbacks clause defines a given method, the normal
runtime error is raised.  The #:defined-table option reports whether a value's
specific implementation supports a method; the presence of a #:fallbacks
implementation for a method does not change this result.
Tests for the #:fallbacks option to define-generics.