Switch from the direct use of slot numbers to handles allocated by
psa_allocate_key.
The general principle for each function is:
* Change `psa_key_slot_t slot` to `psa_key_handle_t handle` or
`psa_key_id_t key_id` depending on whether it's used as a handle to
an open slot or as a persistent name for a key.
* Call psa_create_key() before using a slot, instead of calling
psa_set_key_lifetime to make a slot persistent.
Remove the unit test persistent_key_is_configurable which is no longer
relevant.
Add new functions, psa_load_persistent_key(),
psa_free_persistent_key_data(), and psa_save_persistent_key(), for
managing persistent keys. These functions load to or save from our
internal representation of key slots. Serialization is a concern of the
storage backend implementation and doesn't abstraction-leak into the
lifetime management code.
An initial implementation for files is provided. Additional storage
backends can implement this interface for other storage types.
