Start work on the interpreter

src/libmain.c

@@ -56,7 +56,7 @@ void _GroundListAppend(GroundList* list, GroundValue value);
 
 
 void _GroundInstructionAppend(GroundInstruction* instruction, GroundArg arg);
-void _GroundInstructionExecute(GroundInstruction* instruction, GroundState* state);
+int64_t _GroundInstructionExecute(GroundInstruction* instruction, GroundValue* heap);
 
 
 void _GroundProgramAppend(GroundProgram* program, GroundInstruction instruction);