Add the ExprList.nAlloc column and use it to make the sqlite3ExprListAppend()

routine much faster.

FossilOrigin-Name: 1d3c4662c2f522ac695d97441324069f3fc65f3fa0b87194c7094dfb8cd549f2

src/sqliteInt.h

@@ -2878,6 +2878,7 @@ struct Expr {
 */
 struct ExprList {
   int nExpr;             /* Number of expressions on the list */
+  int nAlloc;            /* Number of a[] slots allocated */
   struct ExprList_item { /* For each expression in the list */
     Expr *pExpr;            /* The parse tree for this expression */
     char *zEName;           /* Token associated with this expression */