Fix Ada 'Modulus attribute

The internal AdaCore test suite pointed out that my earlier patch to
displaying modular types broke the 'Modulus attribute -- it was
off-by-one.

While fixing this, though, I realized that the earlier 'ptype' problem
also affected this attribute.  So, this patch implements a similar fix
in the parser.

This adds some operator+ overloads to gdb_mpz for convenience.  That
class still doesn't have the full complement of operators -- they are
added as needed.