New implementation for constants

VLOCAL expressions keep a reference to their corresponding 'Vardesc',
and 'Upvaldesc' (for upvalues) has a field 'ro' (read-only). So, it is
easier to check whether a variable is read-only. The decoupling in
VLOCAL between 'vidx' ('Vardesc' index) and 'sidx' (stack index)
should also help the forthcoming implementation of compile-time
constant propagation.

lobject.h

@@ -460,6 +460,7 @@ typedef struct Upvaldesc {
   TString *name;  /* upvalue name (for debug information) */
   lu_byte instack;  /* whether it is in stack (register) */
   lu_byte idx;  /* index of upvalue (in stack or in outer function's list) */
+  lu_byte ro;  /* true if upvalue is read-only (const) */
 } Upvaldesc;