SPMV uses some experimental type-casting macros in an effort to remove
endless ifelse clauses for the mixed and extended cases..  It is based
on the code in iskandar's do_test_dot prototypes.  These routines
appear to work OK, and streamlined SPMV's writing.  They will also
appear in HPMV, which is derived from SPMV.

The macro invocations are fairly self-contained, and SPMV can be
switched back to the normal form without undue difficulty.

There is no change in the resulting C code between this form and the
canonical form.