First "complete" implementation of to-be-closed variables

Still missing:
- handling of memory errors when creating upvalue (must run closing
method all the same)
- interaction with coroutines

ltm.h

@@ -40,6 +40,7 @@ typedef enum {
   TM_LE,
   TM_CONCAT,
   TM_CALL,
+  TM_CLOSE,
   TM_N		/* number of elements in the enum */
 } TMS;