Add SP-GiST (space-partitioned GiST) index access method.

SP-GiST is comparable to GiST in flexibility, but supports non-balanced
partitioned search structures rather than balanced trees.  As described at
PGCon 2011, this new indexing structure can beat GiST in both index build
time and query speed for search problems that it is well matched to.

There are a number of areas that could still use improvement, but at this
point the code seems committable.

Teodor Sigaev and Oleg Bartunov, with considerable revisions by Tom Lane