Give a name to a set of functions and mutual recursions that may be used in defretgen.
Usage:
(defretgen my-function-set (a-name (:fnset a-fnset-name) (:function a-function-name) (:mutrec a-mutrec-name)))
The first argument to defretgen is the new name for the set; the second argument is the set of functions, mutual recursions, and preexisting sets that are included in the new set.
