Test foreign function calls with recursive structure by value

Add definition of C functions prodsumpair and doublepairdouble that
take, and for the latter, return, the structure struct-pair+double
(struct_pair_double) that has a structure struct-pair (struct_pair) in
its slot 'pr.  Two tests, fsbv.3 and fsbv.4, have been defined, and
pass.  Fixed argument to libffi-type-pointer call in
libffi-type-pointer :around method so that these will work.