Use the VList object to replace Parse.azVar for tracking the mapping between

SQL parameter names and parameter numbers.  There is a performance
improvement, though there are still a few hiccups in the current code.

FossilOrigin-Name: 68ecafa1425a41358c88f41efea3262f1b4490f2

src/vdbeInt.h

@@ -346,7 +346,6 @@ struct Vdbe {
   Vdbe *pPrev,*pNext;     /* Linked list of VDBEs with the same Vdbe.db */
   Parse *pParse;          /* Parsing context used to create this Vdbe */
   ynVar nVar;             /* Number of entries in aVar[] */
-  ynVar nzVar;            /* Number of entries in azVar[] */
   u32 magic;              /* Magic number for sanity checking */
   int nMem;               /* Number of memory locations currently allocated */
   int nCursor;            /* Number of slots in apCsr[] */
@@ -371,7 +370,7 @@ struct Vdbe {
   char *zErrMsg;          /* Error message written here */
   VdbeCursor **apCsr;     /* One element of this array for each open cursor */
   Mem *aVar;              /* Values for the OP_Variable opcode. */
-  char **azVar;           /* Name of variables */
+  VList *pVList;          /* Name of variables */
 #ifndef SQLITE_OMIT_TRACE
   i64 startTime;          /* Time when query started - used for profiling */
 #endif