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/sync.h

@@ -9,10 +9,10 @@
 #include <condition_variable>
 
 /** Recursive mutex: supports recursive locking, but no waiting */
-typedef std::recursive_mutex CCriticalSection;
+using CCriticalSection = std::recursive_mutex;
 
 /** Plain mutex: supports waiting but not recursive locking */
-typedef std::mutex CWaitableCriticalSection;
+using CWaitableCriticalSection = std::mutex;
 
 #ifdef DEBUG_LOCKORDER
 void EnterCritical(const char* pszName, const char* pszFile, int nLine, void* cs, bool fTry = false);