ARP support

Right now we don't support ARP, we require a hardcoded ARP entry in linux in order to communicate.

verilog-ethernet has an ARP module that we might be able to use, or we can write our own.

We shouldn't need to have an ARP cache since we will only ever be responding. If someone on the other side wants to send out data, it is up to them to send out the ARP requests, we will simply forward them along.