py/qstr: Change type of qstr from mp_uint_t to size_t.

For builds where mp_uint_t is larger than size_t, it doesn't make
sense to use such a wide type for qstrs.  There can only be as many
qstrs as there is address space on the machine, so size_t is the correct
type to use.

Saves about 3000 bytes of code size when building unix/ port with
MICROPY_OBJ_REPR_D.