CIDR/INET improvements

The TODO list contains some items concerning the CIDR/INET datatype.

* %Prevent INET cast to CIDR if the unmasked bits are not zero, or
zero the bits

I added a function for this cast (which zeroes the bits) but then the
opr_sanity regression test failed because there is a cast from INET -> CIDR
but the other way round is considered to be binary compatible.

Actually both types are not binary compatible, since they have a
type component that is either 0 or 1, depending on whether it is of type
INET or CIDR.

If nobody objects, I'll send in a patch that includes a cast function for
the other way round as well.

* Allow INET + INT4 to increment the host part of the address, or
throw an error on overflow

Once at it I wonder how much arithmetic these types need?
What about functions to get/set a specific byte, for example:

inet_get_byte('192.168.1.1'::inet, 0) returns 1
inet_get_byte('192.168.1.1'::inet, 1) returns 1
inet_get_byte('192.168.1.1'::inet, 2) returns 168
inet_get_byte('192.168.1.1'::inet, 3) returns 192

and inet_set_byte('192.168.1.1'::inet, 3, 128) returns '128.168.1.1'

Which other functions have been missing here in the past?

Joachim