Add FxVMFunctionCall class

- This replaces the general extensibility that had existed formerly
  in thingdef_function.cpp. Parameter parsing for function calls is
  shared with state parameter parsing. Functions are defined exactly in
  the same way as action functions, but without the 'action' keyword.

src/zscript/vmbuilder.h

@@ -11,6 +11,7 @@ public:
 	{
 	public:
 		RegAvailability();
+		int GetMostUsed() { return MostUsed; }
 		int Get(int count);			// Returns the first register in the range
 		void Return(int reg, int count);
 		bool Reuse(int regnum);