A generalized mapping macro. Applies fn to the successive values generated
by the arg-specs, analagously to mapcar; that is, on each iteration, each
arg-spec yields one value, and fn is called with these values as arguments.
The values returned by fn are accumulated into a result according to
res-spec. The res-spec is either a list whose car is a predefined result
type, or a list whose car is nil and whose cdr has the same form as the value
of a result type expander (see def-gmap-res-type); or, the res-spec can be
a symbol, which is shorthand for a list of that symbol; or, it can be a list
whose car is :values and whose cdr is a list of result-specs. Similarly,
each of the arg-specs is either a list whose car is a predefined arg type,
or a list whose car is nil and whose cdr has the same form as the value of
an arg type expander (see def-gmap-arg-type).