Make cellSizePtr() a method on the MemPage object, with alternative

implementations depending on the page type.  This results is a small performance
improvement and size reduction.

FossilOrigin-Name: 02f7e9d7d7b93d0b6bbd6cc0d0359b3b741b9931

src/btreeInt.h

@@ -295,6 +295,7 @@ struct MemPage {
   u8 *aDataEnd;        /* One byte past the end of usable data */
   u8 *aCellIdx;        /* The cell index area */
   DbPage *pDbPage;     /* Pager page handle */
+  u16 (*xCellSize)(MemPage*,u8*);  /* cellSizePtr method */
   Pgno pgno;           /* Page number for this page */
 };