Move the persistent storage implementation from psa_key_slot_t to
psa_key_id_t. For the most part, this just means changing the types of
function arguments.
Update the documentation of some functions to reflect the fact that
the slot identifier is purely a storage identifier and is not related
to how the slot is designated in memory.
Add a magic header to the storage format used with files. The
header is used as an initial check that the data is what we expect,
rather than garbage data.
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.
