Create an SP-GiST opclass for inet/cidr.

 Create an SP-GiST opclass for inet/cidr.
 
 This seems to offer significantly better search performance than the
 existing GiST opclass for inet/cidr, at least on data with a wide mix
 of network mask lengths.  (That may suggest that the data splitting
 heuristics in the GiST opclass could be improved.)
 
 Emre Hasegeli, with mostly-cosmetic adjustments by me
 
 Discussion: <CAE2gYzxtth9qatW_OAqdOjykS0bxq7AYHLuyAQLPgT7H9ZU0Cw@mail.gmail.com>