interval / interval -> double operator

Yet another potential addition to the family of operators. Some guy
was asking for it on IRC so...

CREATE OR REPLACE FUNCTION interval_over_interval(interval, interval)
RETURNS float STRICT IMMUTABLE LANGUAGE sql AS $$
SELECT extract(epoch from $1)::float / extract(epoch from $2);
$$;

CREATE OPERATOR /
( leftarg = interval
, rightarg = interval
, procedure = interval_over_interval
);