 Internally use <code>__GMPN_ADD_1</code> etc
     instead of the functions, so they get inlined on all compilers, not just
     gcc and others with <code>inline</code> recognised in gmp.h.
     <code>__GMPN_ADD_1</code> etc are meant mostly to support application
     inline <code>mpn_add_1</code> etc and if they don't come out good for
     internal uses then special forms can be introduced, for instance many
     internal uses are in-place.  Sometimes a block of code is executed based
     on the carry-o