Diagnostic functions

Here's an update of Simon's Heap Page Diagnostic Functions. I now
consider it ready to commit.

The new functions are all under new contrib module,
contrib/pgdiagnostics. I also moved bt_metap, bt_page_items and
bt_page_stats from pgstattuple to the new module.

I copied the bt_* functions almost unmodified. I didn't try to harmonize
the user interface or return values with the new heap page functions,
though that might we worth doing in the future.

I added input value checking to the new functions so that they should
handle invalid data gracefully. All the new functions are nevertheless
superuser-only, just to err on the safe side.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com