Move the `slot_number` field of `psa_key_attributes_t` to
`psa_core_key_attributes_t`. This makes ``psa_core_key_attributes_t` core`
the sole field of `psa_key_attributes_t`. This paves the way to unifying
the two structures.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>