Persistent storage implementation: psa_key_slot_t -> psa_key_id_t

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.
diff --git a/library/psa_crypto_storage_file.c b/library/psa_crypto_storage_file.c
index 03c711a..95857fa 100644
--- a/library/psa_crypto_storage_file.c
+++ b/library/psa_crypto_storage_file.c
@@ -48,15 +48,15 @@
 
 enum { MAX_LOCATION_LEN = sizeof(CRYPTO_STORAGE_FILE_LOCATION) + 40 };
 
-static void key_slot_to_location( const psa_key_slot_t key,
-                                  char *location,
-                                  size_t location_size )
+static void key_id_to_location( const psa_key_id_t key,
+                                char *location,
+                                size_t location_size )
 {
     mbedtls_snprintf( location, location_size,
                       CRYPTO_STORAGE_FILE_LOCATION "psa_key_slot_%d", key );
 }
 
-psa_status_t psa_crypto_storage_load( const psa_key_slot_t key, uint8_t *data,
+psa_status_t psa_crypto_storage_load( const psa_key_id_t key, uint8_t *data,
                                       size_t data_size )
 {
     psa_status_t status = PSA_SUCCESS;
@@ -64,7 +64,7 @@
     size_t num_read;
     char slot_location[MAX_LOCATION_LEN];
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
     file = fopen( slot_location, "rb" );
     if( file == NULL )
     {
@@ -81,12 +81,12 @@
     return( status );
 }
 
-int psa_is_key_present_in_storage( const psa_key_slot_t key )
+int psa_is_key_present_in_storage( const psa_key_id_t key )
 {
     char slot_location[MAX_LOCATION_LEN];
     FILE *file;
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
 
     file = fopen( slot_location, "r" );
     if( file == NULL )
@@ -99,7 +99,7 @@
     return( 1 );
 }
 
-psa_status_t psa_crypto_storage_store( const psa_key_slot_t key,
+psa_status_t psa_crypto_storage_store( const psa_key_id_t key,
                                        const uint8_t *data,
                                        size_t data_length )
 {
@@ -114,7 +114,7 @@
      * affect actual keys. */
     const char *temp_location = CRYPTO_STORAGE_FILE_LOCATION "psa_key_slot_0";
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
 
     if( psa_is_key_present_in_storage( key ) == 1 )
         return( PSA_ERROR_OCCUPIED_SLOT );
@@ -154,12 +154,12 @@
     return( status );
 }
 
-psa_status_t psa_destroy_persistent_key( const psa_key_slot_t key )
+psa_status_t psa_destroy_persistent_key( const psa_key_id_t key )
 {
     FILE *file;
     char slot_location[MAX_LOCATION_LEN];
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
 
     /* Only try remove the file if it exists */
     file = fopen( slot_location, "rb" );
@@ -173,7 +173,7 @@
     return( PSA_SUCCESS );
 }
 
-psa_status_t psa_crypto_storage_get_data_length( const psa_key_slot_t key,
+psa_status_t psa_crypto_storage_get_data_length( const psa_key_id_t key,
                                                  size_t *data_length )
 {
     psa_status_t status = PSA_SUCCESS;
@@ -181,7 +181,7 @@
     long file_size;
     char slot_location[MAX_LOCATION_LEN];
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
 
     file = fopen( slot_location, "rb" );
     if( file == NULL )