(in progress, not for immediate merge!) Process & unix doman socket (named pipe) support

Unlike my last pull request, this perhaps shouldn't be merged in right away, but it adds some important functionality so you may want to take a look at it.

I've added support for libuv pipes (unix domain sockets in case of Linux) to cl-async.
My current goal is implementing usable wrapping for `uv_spawn()`. The code
is currently not tested much (besides running included tests) and process.lisp is in its early stages.