Remove psa_key_slot_t from public headers

This commit marks the beginning of the removal of support for direct
access to key slots. From this commit on, programs that use
psa_key_slot_t will no longer compile.

Subsequent commits will remove the now-unused legacy support in
psa_crypto.c.

include/psa/crypto_platform.h

@@ -46,9 +46,6 @@
 /* PSA requires several types which C99 provides in stdint.h. */
 #include <stdint.h>
 
-/* Integral type representing a key slot number. */
-typedef uint16_t psa_key_slot_t;
-
 /* Integral type representing a key handle. */
 typedef uint16_t psa_key_handle_t;
 

library/psa_crypto.c

@@ -43,6 +43,10 @@
 
 #include "psa/crypto.h"
 
+/* Transitional definition while moving away from directly-accessible key
+ * slots and to a handle-only interface. */
+typedef psa_key_handle_t psa_key_slot_t;
+
 #include "psa_crypto_invasive.h"
 #include "psa_crypto_slot_management.h"
 /* Include internal declarations that are useful for implementing persistently