Updated modbuiltin.c to add conditional support for 3-arg calls to
pow() using MICROPY_PY_BUILTINS_POW3 config parameter. Added support in
objint_mpz.c for for optimised implementation.

The check for division by zero is made by the caller of this function.

Previous to this patch bignum division and modulo would temporarily
modify the RHS argument to the operation (eg x/y would modify y), but on
return the RHS would be restored to its original value.  This is not
allowed because arguments to binary operations are const, and in
particular might live in ROM.  The modification was to normalise the arg
(and then unnormalise before returning), and this patch makes it so the
normalisation is done on the fly and the arg is now accessed as read-only.

This change doesn't increase the order complexity of the operation, and
actually reduces code size.

This patch consolidates the Python logic for division/modulo to one place
within the bignum code.

When DIG_SIZE=32, a uint32_t is used to store limbs, and no normalisation
is needed because the MSB is already set, then there will be left and
right shifts (in C) by 32 of a 32-bit variable, leading to undefined
behaviour.  This patch fixes this bug.

This function computes (x**y)%z in an efficient way.  For large arguments
this operation is otherwise not computable by doing x**y and then %z.

It's currently not used, but is added in case it's useful one day.

For these 3 bitwise operations there are now fast functions for
positive-only arguments, and general functions for arbitrary sign
arguments (the fast functions are the existing implementation).

By default the fast functions are not used (to save space) and instead
the general functions are used for all operations.

Enable MICROPY_OPT_MPZ_BITWISE to use the fast functions for positive
arguments.

Python semantics are that rhs of shift must be non-negative, so there's
no need to handle negative values in the underlying mpz implementation.

This gets int.to_bytes working for bignum, and also struct.pack with 'q'
and 'Q' args on 32-bit machines.

Addresses issue #1155.

See issue #699.

See issue #699.

Addresses issue #1022.

Addresses issue #1027.

msvc does not treat 1L a 64bit integer hence all occurences of shifting it left or right
result in undefined behaviour since the maximum allowed shift count for 32bit ints is 31.
Forcing the correct type explicitely, stored in MPZ_LONG_1, solves this.