Reduce vchan deps; see mirage/ocaml-vchan#53 for corresponding change
Add a virtual conduit-mirage package that pulls in all the depopts

There are now 4 depots for conduit-mirage (dns, vchan, tcpip) and
every upstream package has to specify them.  This package centralised
those with explicit dependencies.
Merge pull request #22 from avsm/reduce-vchan-deps

Reduce vchan deps; see mirage/ocaml-vchan#53 for corresponding change