Update mini-gmp

Update mini-gmp

* src/mini-gmp.c, src/mini-gmp.h: Copy from GMP 6.2.0.
This incorporates:
2019-12-05 remove some sizeof(mp_limb_t)
2019-12-04 (mpn_invert_3by2): Remove special code for limb sizes
2019-12-04 (mpn_invert_3by2): Limit size of an intermediate
2019-11-20 (mpn_invert_3by2): Use xor instead of negation
2019-11-19 (mpn_invert_3by2): Move an assert earlier
2019-11-19 (mpn_invert_3by2): Add a new shortcut
2019-11-17 Prepend "unsigned" to MINI_GMP_LIMB_TYPE
2019-11-17 Enable testing with different limb sizes (types)
2019-11-20 Use already defined constants
2019-11-09 Avoid undefined behaviour with small limb sizes