Add `IpNetwork::with_netmask_checked` & `Ipv{4,6}Network::with_netmask_checked` #216

MarkusPettersson98 wants to merge 3 commits intoachanda:masterachanda/ipnetwork:masterfrom

MarkusPettersson98:const-with_netmaskMarkusPettersson98/ipnetwork:const-with_netmaskCopy head branch name to clipboard