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/Blocks/BlockMobSpawner.h

@@ -15,14 +15,9 @@ class cBlockMobSpawnerHandler:
 
 public:
 
-	cBlockMobSpawnerHandler(BLOCKTYPE a_BlockType):
-		Super(a_BlockType)
-	{
-	}
-
-
-
+	using Super::Super;
 
+private:
 
 	virtual bool OnUse(
 		cChunkInterface & a_ChunkInterface,
@@ -31,7 +26,7 @@ public:
 		const Vector3i a_BlockPos,
 		eBlockFace a_BlockFace,
 		const Vector3i a_CursorPos
-	) override
+	) const override
 	{
 		return a_ChunkInterface.UseBlockEntity(&a_Player, a_BlockPos.x, a_BlockPos.y, a_BlockPos.z);
 	}
@@ -40,7 +35,7 @@ public:
 
 
 
-	virtual bool IsUseable() override
+	virtual bool IsUseable() const override
 	{
 		return true;
 	}
@@ -49,7 +44,7 @@ public:
 
 
 
-	virtual cItems ConvertToPickups(NIBBLETYPE a_BlockMeta, cBlockEntity * a_BlockEntity, const cEntity * a_Digger, const cItem * a_Tool) override
+	virtual cItems ConvertToPickups(NIBBLETYPE a_BlockMeta, cBlockEntity * a_BlockEntity, const cEntity * a_Digger, const cItem * a_Tool) const override
 	{
 		// No pickups
 		return {};
@@ -64,7 +59,7 @@ public:
 		cPlayer & a_Player,
 		Vector3i a_BlockPos,
 		BLOCKTYPE a_OldBlockType, NIBBLETYPE a_OldBlockMeta
-	) override
+	) const override
 	{
 		auto handler = a_Player.GetEquippedItem().GetHandler();
 		if (!a_Player.IsGameModeSurvival() || !handler->CanHarvestBlock(E_BLOCK_MOB_SPAWNER))