Mostly just a global search and replace.  Except rt_is_true which
becomes mp_obj_is_true.

Still would like to tidy up some of the names, but this will do for now.

Made solely to unbreak int-long.py test which in turn uncovered thinko
with implementation of inplace ops. On mpz level, bitwise ops implemented
only for same-sign numbers, and are not efficient (unconditional calling of
mpn_cmp() is apparently superfluous).

Fixed modulo operator on ints and mp ints to agree with python. Added intdivmod.c and tests/basics/modulo.py.

Remove unnecessary includes. Add includes that improve portability.

If operation will overflow, a multi-precision integer is created.

Implement not, shl and shr in mpz library.  Add function to create mpzs
on the stack, used for memory efficiency when rhs is a small int.
Factor out code to parse base-prefix of number into a dedicated function.

Some functionality is still missing (eg and, or, bit shift), and some
things are buggy (eg subtract).