Further linux_sll support

[JulianSchmid]

Followup from #99

Specifically, some points missing are:

• Implementation of PacketHeaders::from_linux_sll()
• Implementation of LaxSlicedPacket::from_linux_sll()
• Implementation of LaxPacketHeaders::from_linux_sll()
• Revising the added test code, making sure it makes sense and the relevant cases are covered