Heap page diagnostic functions

I'm taking over Simon's heap page diagnostic functions to get them into
shape for committing.

I'm thinking of having a new contrib-module for them: pgforensics. All
the new functions go there, and I'm also going to move bt_page_items,
bt_page_stats, and bt_metap functions from pgstattuple to the new
module. They're all low-level functions that no application should be
calling, so there shouldn't be any compatibility issues in moving them.

That'd leave only the functions that a DBA or tools should be concerned
with in pgstattuple, and all the low-level debugging aids to the new
module. Thoughts?

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