Stack reallocation done with a single realloc

To avoid the need of both the old and the new stack addresses valid
at the same time, to correct the pointers to the stack, these pointers
are changed to offsets before the reallocation and then changed back
to pointers after the reallocation.

lobject.h

@@ -158,8 +158,13 @@ typedef union StackValue {
 typedef StackValue *StkId;
 
 
+/*
+** When reallocating the stack, change all pointers to the stack into
+** proper offsets.
+*/
 typedef union {
   StkId p;  /* actual pointer */
+  ptrdiff_t offset;  /* used while the stack is being reallocated */
 } StkIdRel;
 
 
@@ -626,6 +631,7 @@ typedef struct UpVal {
   lu_byte tbc;  /* true if it represents a to-be-closed variable */
   union {
     TValue *p;  /* points to stack or to its own value */
+    ptrdiff_t offset;  /* used while the stack is being reallocated */
   } v;
   union {
     struct {  /* (when open) */