Making Async play better with others

[Stephen Weeks]

> Stephen, what do you think about Markus' proposal for how to organize
> it?

I am highly skeptical of the approach of propagating Linux submodules and
conditional compilation throughout the codebase.  I think it would be preferable
to design a platform-agnostic interface at the OCaml/C boundary, implemented on
the C side using platform-specific C files with minimal conditional compilation
at the C level and no conditional compilation at the OCaml level.  Platforms
that are unable to implement particular aspects of the interfaces should provide
functions that raise exceptions.  Then all OCaml code compiles everywhere, and
it one gets on each platform the maximum ability to run what is actually
implemented.