py/dynruntime.h: Implement MP_OBJ_NEW_QSTR.

Because mpy_ld.py doesn't know the target object representation, it emits
instances of `MP_OBJ_NEW_QSTR(MP_QSTR_Foo)` as const string objects, rather
than qstrs. However this doesn't work for map keys (e.g. for a locals dict)
because the map has all_keys_are_qstrs flag is set (and also auto-complete
requires the map keys to be qstrs).

Instead, emit them as regular qstrs, and make a functioning MP_OBJ_NEW_QSTR
function available (via `native_to_obj`, also used for e.g. making
integers).

Remove the code from mpy_ld.py to emit qstrs as constant strings, but leave
behind the scaffold to emit constant objects in case we want to do use this
in the future.

Strictly this should be a .mpy sub-version bump, even though the function
table isn't changing, it does lead to a change in behavior for a new .mpy
running against old MicroPython. `mp_native_to_obj` will incorrectly return
the qstr value directly as an `mp_obj_t`, leading to unexpected results.
But given that it's broken at the moment, it seems unlikely that anyone is
relying on this, so it's not work the other downsides of a sub-version bump
(i.e. breaking pure-Python modules that use @native). The opposite case of
running an old .mpy on new MicroPython is unchanged, and remains broken in
exactly the same way.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

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