[CM] fastest way to pass block of samples in and out of s7

[bil at ccrma.Stanford.EDU]

Return the value of s7_make_float_vector_wrapper
to scheme, so some scheme variable has its value,
just like s7_make_float_vector.  Then treat it like
a normal float-vector (i.e. use float-vector-ref and
so on).  The gc_protects in the C code were paranoia
about complicated functions like s7_is_equivalent
when the values in question were not alive in scheme.