New implementation for 'tbclist'

- Fixes a bug, by removing dummy nodes together with the node
itself. (The previous implementation could leave dummy nodes in frames
which otherwise had no tbc variables, and therefore would not close
variables; that could leave 'tbclist' pointing higher than 'top', which
could dangle if the stack shrank.)

- Computes MAXDELTA based on the type of delta, to ease changing its
type if needed.

- Instead of 'isdummy', uses 'delta==0' to signal dummy nodes. (Dummy
nodes always have MAXDELTA for their real delta.)

lobject.h

@@ -139,13 +139,14 @@ typedef struct TValue {
 ** Entries in a Lua stack. Field 'tbclist' forms a list of all
 ** to-be-closed variables active in this stack. Dummy entries are
 ** used when the distance between two tbc variables does not fit
-** in an unsigned short.
+** in an unsigned short. They are represented by delta==0, and
+** their real delta is always the maximum value that fits in
+** that field.
 */
 typedef union StackValue {
   TValue val;
   struct {
     TValuefields;
-    lu_byte isdummy;
     unsigned short delta;
   } tbclist;
 } StackValue;