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.
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 1a038a1..bff0ce7 100644
--- a/library/psa_crypto.c
+++ b/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