Add ARP module

[lachrimae]

I'd like to parse ARP packets for Ethernet+IPv4. Might you be interested in merging this if I complete the implementation?

[codecov]

Codecov Report

Merging #15 (cab1b5b) into master (1d2be79) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #15   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files          45       45           
  Lines       16320    16320           
=======================================
  Hits        16271    16271           
  Misses         49       49           

Impacted Files	Coverage Δ
src/link/mod.rs	100.00% <ø> (ø)

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[ravenexp]

Hi! I'm interested in using etherparse for parsing and building ARP packets.

My current implementation uses Ethernet2Header and a hand-coded ARP parser/builder.
This PR contains a higher quality implementation that I'd like to adopt.

I somewhat disagree on the implementation approach though: there is no such thing as "ARP header" according to RFC 826. ARP defines fixed-length Ethernet packets that have a very simple structure. Breaking it down further into "ARP header" and "ARP payload" seems artificial to me.

Should I try to submit an independent PR or are you rather interested in merging the current one?

[lachrimae]

@ravenexp I am pretty happy for you to submit your own version of the ARP module with the improvements you have in mind. I haven't heard back from the maintainer on this thread so I'm not sure what they think.