ecific macros the same way as CPU specific assembler code.
     Code used would no longer depend on cpp predefines, and the current
     nested conditionals could be flattened out.
<li> <code>mpz_get_si</code> returns 0x80000000 for -0x100000000, whereas it's
     sort of supposed to return the low 31 (or 63) bits.  But this is
     undocumented, and perhaps not too important.
<li> <code>mpz_init_set*</code> and <code>mpz_realloc</code> could allocate
     say an extra 16 limbs over what's needed, so as to 