Proof of concept for the ability to use the expression columns in an index

on expressions in place of equivalent expressions in the result set or in
the WHERE clause.  This check-in compiles but is mostly untested.

FossilOrigin-Name: a52ef2ad7c0e14b78b801f16a1f6ea8d8fa9ae5d7d810e18dd24c600b662a312

src/sqliteInt.h

@@ -3324,6 +3324,7 @@ struct Walker {
     struct CCurHint *pCCurHint;                /* Used by codeCursorHint() */
     int *aiCol;                                /* array of column indexes */
     struct IdxCover *pIdxCover;                /* Check for index coverage */
+    struct IdxExprTrans *pIdxTrans;            /* Convert indexed expr to column */
   } u;
 };