Implement OBJSWAP instruction

instructions.c

@@ -196,7 +196,7 @@ const struct instruction instructions[NR_OPCODES] = {
         OP(FTOS, 0),
         OP(S_MOD, 0),
         OP(S_PLUSA2, 0),
-        OP(OBJSWAP, 0), // TODO
+        OP(OBJSWAP, 0),
         OP(S_ERASE, 0), // TODO
         OP(SR_REF2, 0), // TODO
         OP(S_ERASE2, 0),

vm.c

@@ -751,6 +751,15 @@ static void execute_instruction(enum opcode opcode)
 		heap_ref(stack_pop().i);
 		break;
 	}
+	case OBJSWAP: {
+		stack_pop(); // type
+		union vm_value *b = stack_pop_var();
+		union vm_value *a = stack_pop_var();
+		union vm_value tmp = *a;
+		*a = *b;
+		*b = tmp;
+		break;
+	}
 	//
 	// --- Control Flow ---
 	//