Start holding mp_int * in P6bigint.
Introduce union for holding smallint in P6bigint.

Not using the smallint part yet.
Teach a few things to handle smallint case.

We never actually create that case yet, so really we can only test the
checking code doesn't get false positives so far.
Make get_bigint complain if it sees a smallint.
Start producing smallint for empty and box cases.

At this point, everything fails in an orderly way, with "Incomplete
smallint handling!". The task from here is just to fix things until
we never see this error again. :-)
Handle smallint in bigint to/from string.
Fix "fits in 32 bits" test.
Add smallint handling to various basic ops.

Gets us up to 8 passing tests in NQP's 60-bigint.t. Also adds much of
the infrastructure we will need to do the rest.
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.
remove some stray backslashes