When nil IP is returned from net.ParseIP, it is accepted into Inet type,
but not properly marked as being Null. That introduces issues later on
when calling for example EncodeBinary, since it does not assume this can
happen.
This commit resolves that by properly detecting zero-length net.IP and
setting status to Null if that is the case.
Set having the capability to assign an object of the same type was
inconsistently implemented. Some places it was not implemented at all, some
places it was a shallow copy, some places a deep copy. Given that it doesn't
seem likely to ever be used, and if it is needed it is easy enough to do
outside of the library this code has been removed.
Though this doesn't follow Go naming conventions exactly it makes names more
consistent with PostgreSQL and it is easier to read. For example, TIDOID becomes
TidOid. In addition this is one less breaking change in the move to V3.
