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

src/include/access/gin_private.h

@@ -24,6 +24,10 @@
  * Note: GIN does not include a page ID word as do the other index types.
  * This is OK because the opaque data is only 8 bytes and so can be reliably
  * distinguished by size.  Revisit this if the size ever increases.
+ * Further note: as of 9.2, SP-GiST also uses 8-byte special space.  This is
+ * still OK, as long as GIN isn't using all of the high-order bits in its
+ * flags word, because that way the flags word cannot match the page ID used
+ * by SP-GiST.
  */
 typedef struct GinPageOpaqueData
 {