[netstack3] Add reference counted HashMap
[netstack3] Verify behavior when attempting to forward IPv6 packets w/ forwarding disabled
[netstack3] Do not forward loopback addresses
[netstack3] Split IpProto into Ipv4Proto and Ipv6NextHeader
[netstack3] Send gratuitous ARP when a device is brought up
[netstack3] Fuzz parsers and serializers
[netstack3] ARP: Add tests for ARP on a broadcast medium
[netstack3] Make sure we properly handle unrecognized IPv6 next header values
[netstack3] Migrate all transport protocols to trait associated types
[netstack3] Implement InterfaceIndexToName and InterfaceNameToIndex