Process, unix domain socket (named pipe) and filesystem event 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.

Most important change: I've added general STREAMISH class which is wrapper for
uv streams (named so due to lack of better possibilities, uvstream used in the code
to refer to underlying handles, and it's not a lisp stream so calling it a stream may
be confusing). SOCKET is base class for TCP-SOCKET and PIPE, SOCKET-SERVER
is base class for TCP-SERVER and PIPE-SERVER.

Pipes are useful by themselves but they're also important for process I/O redirection.