Revamped concurrency API

[MIURA Masahiro]

Jeremie Pelletier wrote:
> I also don't believe one model is "ruling them all".

Let me clarity this, just in case I have caused an unnecessary
confusion:  I think Sean's Erlang-like API is meant to coexist
with the current core.thread, and that you can use one or both
of them to build higher-level concurrency models.

I think it's nice to have core.thread *and* message-passing API
in Phobos.  Thread alone is too primitive for daily use, but
we don't want to have too many concurrency models.