* fix ipv6 case for cidr.Class
Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* add check and test case for invalid ipv6 cidr
Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* net package is bad at detecting ipv6/ipv4
Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* rename Class -> Split
Signed-off-by: Chris O'Haver <cohaver@infoblox.com>