Add an experimental location(X) SQL function that attempt to return the

location of the payload within the database for the record that contains
column X.  location(X) returns NULL if X is not an ordinary table column or
if SQLite cannot figure out the location because it is using a covering index.

FossilOrigin-Name: 51be9558164301c5dd4df23ab8b3e67de0b522f8d36f79f3d84d45d3dc2a83a4

src/btree.h

@@ -291,6 +291,7 @@ int sqlite3BtreeNext(BtCursor*, int flags);
 int sqlite3BtreeEof(BtCursor*);
 int sqlite3BtreePrevious(BtCursor*, int flags);
 i64 sqlite3BtreeIntegerKey(BtCursor*);
+i64 sqlite3BtreeLocation(BtCursor*);
 int sqlite3BtreePayload(BtCursor*, u32 offset, u32 amt, void*);
 const void *sqlite3BtreePayloadFetch(BtCursor*, u32 *pAmt);
 u32 sqlite3BtreePayloadSize(BtCursor*);