A number of things in one commit:

1) Added a "*showmem" "wizard" command to get a dump of all memory
   being used by tiles in the database.
2) Made a slight correction to the way magic detects exact overlap
   of instances of the same cell.  This probably does not make any
   actual difference in practice.
3) Corrected an uninitialized variable in dbReComputeBboxFunc().
4) Changes DBSrCellPlaneArea() to use a static BPEnum variable, so
   that it does not waste time allocating and freeing memory for
   the same thing over and over again.
5) Corrected a memory leak in the tech file "extract" section that
   loses memory every time the extraction style is changed.
6) Corrected the tile join routines to fix a bad memory leak in the
   tile allocation and recovery---a fix which was mentioned in issue
   #414 but which had not yet been implemented.  This has now been
   tested and confirmed to work.

database/databaseInt.h

@@ -194,6 +194,8 @@ typedef struct
 extern int dbNumSavedRules;
 extern Rule dbSavedRules[];
 
+extern HashTable dbCellDefTable;	/* Exported for diagnostics */
+
 /* -------------------- Internal procedure headers -------------------- */
 
 extern void DBUndoPutLabel();