compress method for spgist

[Teodor Sigaev]

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/***@sss.pgh.pa.us
http://www.postgresql.org/message-id/***@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.