Gmp: Disable Gmp.smpz()->invert().

This function has multiple issues:

  * It doesn't work as implemented (eg argument 6 to mpb_sec_invert()
    is wrong).

  * It would clobber THIS. Gmp manual 8.1:
    "In either case, the input A is destroyed."

  * To work, the number of limbs in THIS, modulo and res MUST
    be the same (aka n). This can probably be accomplished
    by using mpz_realloc2(), of which the Gmp manual 5.1 says:
    "Calling this function is never necessary; reallocation is
    handled automatically by GMP when needed."

Fixing the above issues while still keeping the _sec property
is non-trivial, and best left to the Gmp people, so we wait for
an mpz_invert_sec().