Import/Export to limb data; todo: implement auto-allocation of export destination array.
Auto-allocating version of the Export.  Completed MPIR 2.6.0 section 5.14.
Size and GetLimb. Completed MPIR 2.6.0 section 5.16.
MPIR 2.6.0 section 9 - random state init/seed/clear/generate_ui