CHANGES:

* split opam package into two separate ones: a core
  `arp` package and the `arp-mirage` implementation
  for MirageOS that has more dependencies.  This
  eliminates the use of depopts that was done previously
  to build the Mirage layer. (mirage/arp#7 @avsm)

* port build system to Dune (mirage/arp#7 @avsm). The `make coverage`
  and `make bench` targets will do the job of the previous
  topkg targets for those.

* minor fixes to ocamldoc comments to be compatible with
  odoc.

* use mirage-random and mirage-random-test instead of a
  nocrypto dependency in tests and bench (mirage/arp#7 @hannesm)

* import tests from mirage-tcpip (mirage/arp#8 @hannesm)

* depend on the ethernet opam package, no longer provided
  by tcpip >3.7.0 (mirage/arp#9 @hannesm)