BlockHandler initialisation is a constant expression (#4891)

* BlockHandler initialisation is a constant expression

If we can't make it all namespaces, this is the next best I guess.

+ Tag handlers constexpr, const as needed
+ Inherit constructors
* Privatise handler functions

* More constexpr

Co-authored-by: Alexander Harkness <me@bearbin.net>

src/Simulator/FloodyFluidSimulator.cpp

@@ -299,8 +299,7 @@ void cFloodyFluidSimulator::SpreadToNeighbor(cChunk * a_NearChunk, int a_RelX, i
 	// Wash away the block there, if possible:
 	if (CanWashAway(BlockType))
 	{
-		cBlockHandler * Handler = BlockHandler(BlockType);
-		if (Handler->DoesDropOnUnsuitable())
+		if (cBlockHandler::For(BlockType).DoesDropOnUnsuitable())
 		{
 			m_World.DropBlockAsPickups(absPos, nullptr, nullptr);
 		}