Remove hdr_raw usage

[jamesmunns]

A retooling of how we use headers in send_raw operations. Major changes:

• NetStack::send_raw now takes HeaderSeq instead of Header
• NetStack::send_raw no longer takes hdr_raw, as well as many other parts of the system
• Reduce usage of CommonHeader and AnyAllAppendix, instead using HeaderSeq more consistently
• Headers are now generally re-serialized down at the InterfaceSink level, so they don't need to be re-encoded more than once.

Originally, the idea behind passing around hdr_raw was to avoid cases where profiles/interfaces or borrowed sockets would need to re-serialize the header when forwarding. However, there are places in the code where the header may be modified, particularly for broadcast flooding, or even for updating the TTL count when forwarding, meaning that the serialized and in-memory versions could get out of sync with each other.

To avoid this, we no longer pass around the "raw" header, and just demand it is re-serialized when necessary. This is actually likely not very burdensome at all, as the max header size is 28 bytes, which should be trivial to do.