py: Implement bit-shift and not operations for mpz.

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.

py/parsenumbase.h

@@ -0,0 +1 @@
+int mp_parse_num_base(const char *str, uint len, int *base);