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/backend/access/spgist/spgscan.c

@@ -658,6 +658,7 @@ Datum
 spgcanreturn(PG_FUNCTION_ARGS)
 {
 	Relation	index = (Relation) PG_GETARG_POINTER(0);
+	/* int			i = PG_GETARG_INT32(1); */
 	SpGistCache *cache;
 
 	/* We can do it if the opclass config function says so */