Add WIP Mirage_conduit module
Add a static services lookup from `uri.services` for Unix.
Add a `Mirage_resolver` module

There is a static version that uses built-in information, and a
functor that spawns a resolver against a network stack
Hook Mirage into the build
Allow the Mirage_conduit resolver to be specified explicitly
Add conduit mirage connector
Improve the Mirage test case
+gitignore
Async depends on `ipaddr.unix` as well
Add a `Mirage_conduit.S` module type
Remove dependency on `Conduit` from Async
Remove the Client/Server variants from the Conduit interface.

They should instead live in the backends, where they can vary
depending on the specific interpretations desired.
Adapt to moving the type definitions into the backends.

Temporarily unhooks the resolver interfaces in favour of an explicit
connection-management interface.
Rename all the modules to be prefixed by `Conduit_` to follow cohttp

This follows the same convention as Cohttp, but also helps ensure
there's no module namespace clash with generic names such as
`Lwt_unix_net` that other packages might use.
Lift the end point resolution into Lwt_unix directly
Expose sexp accessors for flows, so tehy can remain abstract
remove debugging accidentally committed
Make the Conduit unix src optional to avoid an unnecessary bind on every connect
Add `Conduit_resolver_mirage.localhost` as a dependency-free default
make the stack optional in Conduit_mirage to provide a default