add and use constant time 32 byte equality function

src/device/device_ledger.cpp

@@ -136,7 +136,8 @@ namespace hw {
     }
 
     bool operator==(const crypto::key_derivation &d0, const crypto::key_derivation &d1) {
-      return !memcmp(&d0, &d1, sizeof(d0));
+      static_assert(sizeof(crypto::key_derivation) == 32, "key_derivation must be 32 bytes");
+      return !crypto_verify_32((const unsigned char*)&d0, (const unsigned char*)&d1);
     }
 
     /* ===================================================================== */