C++20 Round 5: typedef -> using, IMPLEMENT_SERIALIZE macro cleanup

- Convert 15 typedef declarations to C++11 using aliases across 12 files:
  script.h (valtype, CTxDestination), serialize.h (CSerializeData),
  keystore.h (KeyMap, ScriptMap, CryptedKeyMap), sync.h (CCriticalSection,
  CWaitableCriticalSection), sync.cpp (LockStack), key.h (CPrivKey, CSecret),
  crypter.h (CKeyingMaterial), allocators.h (SecureString), main.h (MapPrevTx),
  wallet.h (mapValue_t, removed duplicate), miner.cpp (TxPriority),
  kernel.cpp (MapModifierCheckpoints)
- Remove redundant duplicate mapValue_t typedef in wallet.h
- IMPLEMENT_SERIALIZE macro: replace assert() warning suppression with
  [[maybe_unused]] attributes on fGetSize/fWrite/fRead/nSerSize

src/kernel.cpp

@@ -14,7 +14,7 @@ extern unsigned int nTargetSpacing;
 // Set to 20-minute for production network
 //unsigned int nModifierInterval = MODIFIER_INTERVAL;
 
-typedef std::map<int, unsigned int> MapModifierCheckpoints;
+using MapModifierCheckpoints = std::map<int, unsigned int>;
 
 // Hard checkpoints of stake modifiers to ensure they are deterministic
 static std::map<int, unsigned int> mapStakeModifierCheckpoints = {