Update dependencies again, attempt to re-enable appveyor

[djs55]

Update all the dependencies to latest, mostly minor changes:

• Cstruct.len is renamed Cstruct.length
• Fmt.strf is renamed Fmt.str
• mirage-protocols has been deprecated in favour of using libraries directly (Tcpip, Arp), removing a layer of indirection

The most significant change is the removal of the ~listeners argument on the TCP input. This has been replaced with a listen and unlisten. Currently I've not added any calls to unlisten so it's probably leaking.

I've added an appveyor.yml based on mirage/mirage-block, to see if the CI build can be restored.

The minimum OCaml version needed by dependencies is now 4.08.

The remaining Windows package pins correspond to open PRs which I'd like to merge and release as soon as possible.

[djs55]

CI is green! Let's merge this to make master green too!