compress method for spgist

When we developed SP-GiST we missed analogue of GiST's compress method. There 
was two reasons for that: lack of imagination to imagine case with different 
types of indexed value and column, and we didn't want call some method while 
index fit in one page. Some discussion on that
http://www.postgresql.org/message-id/27542.1323534819@sss.pgh.pa.us
http://www.postgresql.org/message-id/4EE77EA1.6030503@sigaev.ru

But we was wrong: PostGIS guys found an example: polygon indexing with storing 
just a bounding box. Actually, I don't know index structure for boxes suitable 
for SP-GiST but I'm not a geometer. They are.

Attached patch provides support of optional compress method for SP-GiST.

-- 
Teodor Sigaev                                   E-mail: teodor@sigaev.ru
                                                    WWW: http://www.sigaev.ru/