inet/cidr

I was looking at upgrading to 8.2, but I make extensive use of the IP4
module. It doesn't make properly due to the changes in the inet/cidr
types, notably the removal of the type (is_cidr) field of the
inet_struct, which the module uses when casting: ip4_cast_to_cidr,
ip4r_cast_to_cidr, and a check in ip4r_cast_from_cidr.

I read a bit about the cidr/inet types becoming binary compatible, but
I wasn't sure if I needed to do more extensive conversion, or if I
could just remove the references to the type field and be happy.

-Mike