py: Shrink mp_arg_t struct by using reduced-size integer members.
qstrs ids are restricted to fit within 2 bytes already (eg in persistent bytecode) so it's safe to use a uint16_t to store them in mp_arg_t. And the flags member only needs a maximum of 2 bytes so can also use uint16_t. Savings in code size can be significant when many mp_arg_t structs are used for argument parsing. Eg, this patch reduces stmhal by 480 bytes.