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.