Moves non-lwt code out of lwt submodule

The code that I've moved out converts between our SDN abstraction layer
and OpenFlow 1.0/1.3. None of it is Lwt-specific and can be used by
async too.

frenetic builds unchanged with this refactoring, as it is all internal.
Merge pull request #54 from frenetic-lang/abstraction-layer-common-elements

Moves non-lwt code out of lwt submodule