Add support for index-only scans in GiST.

This adds a new GiST opclass method, 'fetch', which is used to reconstruct
the original Datum from the value stored in the index. Also, the 'canreturn'
index AM interface function gains a new 'attno' argument. That makes it
possible to use index-only scans on a multi-column index where some of the
opclasses support index-only scans but some do not.

This patch adds support in the box and point opclasses. Other opclasses
can added later as follow-on patches (btree_gist would be particularly
interesting).

Anastasia Lubennikova, with additional fixes and modifications by me.

src/include/access/gist.h

@@ -33,7 +33,8 @@
 #define GIST_PICKSPLIT_PROC				6
 #define GIST_EQUAL_PROC					7
 #define GIST_DISTANCE_PROC				8
-#define GISTNProcs						8
+#define GIST_FETCH_PROC					9
+#define GISTNProcs					9
 
 /*
  * strategy numbers for GiST opclasses that want to implement the old