py/obj: Add new type flag to indicate subscr accepts slice-on-stack.

The recently merged 5e9189d6d1c00c92694888bf9c74276779c40716 now allows
temporary slices to be allocated on the C stack, which is much better than
allocating them on the GC heap.

Unfortunately there are cases where the C-allocated slice can escape and be
retained as an object, which leads to crashes (because that object points
to the C stack which now has other values on it).

The fix here is to add a new `MP_TYPE_FLAG_SUBSCR_ALLOWS_STACK_SLICE`.
Native types should set this flag if their subscr method is guaranteed not
to hold on to a reference of the slice object.

Fixes issue #17733 (see also #17723).

Signed-off-by: Damien George <damien@micropython.org>

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