Support statistics for RUM

[za-arthur]

From #15.
Estimated row count is not good:

explain (analyze) select * from rum where tsvector @@ 'cow'::tsquery order by id <=> (10^6)::int limit 10;
                                                                QUERY PLAN                                                                 
-------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=12.00..13.43 rows=10 width=56) (actual time=21.027..21.031 rows=10 loops=1)
   ->  Index Scan using rum_tsvector_id_idx on rum  (cost=12.00..7171.81 rows=50125 width=56) (actual time=21.024..21.028 rows=10 loops=1)
         Index Cond: (tsvector @@ '''cow'''::tsquery)
         Order By: (id <=> 1000000)
 Planning time: 0.181 ms
 Execution time: 21.274 ms