py/objint: Try to convert big-int back to small-int after binary op.

Before this change, long/mpz ints propagated into all future calculations,
even if their value could fit in a small-int object.  With this change, the
result of a big-int binary op will now be converted to a small-int object
if the value fits in a small-int.

For example, a relatively common operation like `x = a * b // c` where
a,b,c all small ints would always result in a long/mpz int, even if it
didn't need to, and then this would impact all future calculations with
x.

This adds +24 bytes on PYBV11 but avoids heap allocations and potential
surprises (e.g. `big-big` is now a small `0`, and can safely be accessed
with MP_OBJ_SMALL_INT_VALUE).

Performance tests are unchanged on PYBV10, except for `bm_pidigits.py`
which makes heavy use of big-ints and gains about 8% in speed.

Unix coverage tests have been updated to cover mpz code that is now
unreachable by normal Python code (removing the unreachable code would lead
to some surprising gaps in the internal C functions and the functionality
may be needed in the future, so it is kept because it has minimal
overhead).

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

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