Add index-only scan support to btree_gist.

inet, cidr, and timetz indexes still cannot support index-only scans,
because they don't store the original unmodified value in the index, but a
derived approximate value.

contrib/btree_gist/expected/float4.out

@@ -74,10 +74,10 @@ SELECT count(*) FROM float4tmp WHERE a >  -179.0::float4;
 
 EXPLAIN (COSTS OFF)
 SELECT a, a <-> '-179.0' FROM float4tmp ORDER BY a <-> '-179.0' LIMIT 3;
-                  QUERY PLAN                   
------------------------------------------------
+                     QUERY PLAN                     
+----------------------------------------------------
  Limit
-   ->  Index Scan using float4idx on float4tmp
+   ->  Index Only Scan using float4idx on float4tmp
          Order By: (a <-> (-179)::real)
 (3 rows)