sorting operators in pg_dump

Currently, pg_dump sorts operators by name, but operators with the same
name come out in random order.  A few releases ago we adjusted this for
functions, so that they are in increasing number of arguments order.
I'd like to do this for operators as well, so that they come out in
order, say, prefix, postfix, infix.

(It might be surprising that something like this is necessary, but it
happens.  ip4r for example contains operators with different fixnesses
(fixities?).)

Patch attached, and a little test case.