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/allocators.h

@@ -254,7 +254,7 @@ struct zero_after_free_allocator : public std::allocator<T>
 };
 
 // This is exactly like std::string, but with a custom allocator.
-typedef std::basic_string<char, std::char_traits<char>, secure_allocator<char> > SecureString;
+using SecureString = std::basic_string<char, std::char_traits<char>, secure_allocator<char>>;
 
 static inline SecureString MakeSecureString(const std::string& value)
 {