New header crypto_sizes.h

This header will contain macros that calculate buffer sizes, whose
semantics are standardized but whose definitions are
implementation-specific because they depend on the available algorithms
and on some permitted buffer size tolerances.

Move size macros from crypto_struct.h to crypto_sizes.h, because these
definitions need to be available both in the frontend and in the
backend, whereas structures have different contents.
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index dcf1ba2..8f3a7f0 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -2070,6 +2070,10 @@
 }
 #endif
 
+/* The file "crypto_sizes.h" contains definitions for size calculation
+ * macros whose definitions are implementation-specific. */
+#include "crypto_sizes.h"
+
 /* The file "crypto_struct.h" contains definitions for
  * implementation-specific structs that are declared above. */
 #include "crypto_struct.h"