Make mantissa type of mp_no configurable

Make mantissa type of mp_no configurable

The mantissa of mp_no is intended to take only integral values.  This
is a relatively good choice for powerpc due to its 4 fpus, but not for
other architectures, which suffer due to this choice.  This change
makes the default mantissa a long integer and allows powerpc to
override it.  Additionally, some operations have been optimized for
integer manipulation, resulting in a significant improvement in
performance.