IP-stack horror: proper isolation between layers

[alfreb]

We've now finally purged ethernet from higher level protocols. The main issue was to decide on how to hide / expose information in a packet as it traverses the stack. Essentially now ethernet is in charge of enforcing the isolation by hiding it's own header when pushing upstream, and revealing when it receives downstream. All headers above link-layer, including arp, will have their buffers start after the ethernet header. All packet-level protocols above IP still needs to know about the IP header so no need for further isolation there afaik.

Most of this work is done by changing the Packet class to have 3 data members, all pointers:

• buffer start (actual buffer start)
• layer start (e.g. buffer start relative to the protocol)
• buffer end. Corresponds to end of buffer / capacity for all protocols.

Lots of other fixes / changes here as well, notably an experimental packet wrapper in ICMPv4 - this is an alternative to inheriting IP-packet / Packet which can be useful if you need to temporarily add data members. Arp was also heavily modified. etc.

I expect issues and will fix.