teach bigint_div about smallint.
implement smallint support for left-shift
smallint support in right shift
implementations for or, xor and and with smallint
using force_bigint on the result is wrong.
forgot an mp_init in bigint_shr
abs and neg now know smallint via MVM_BIGINT_UNARY_OP
implement bitwise not with smallint support
smallint support is not so simple for pow, so leave it be for now
from_num handles smallint now.
expmod will just use bigints everywhere for now.