Revamped concurrency API

[MIURA Masahiro]

Sean Kelly wrote:
> void sendmsg(T)( Pid pid, T val );
> final void recvmsg(T...)( Pid pid, T ops );
> Pid spawn(T)( T fun );
> 
> spawn() is pretty limited so far in that it only spawns threads--I'd
> expect that function to end up with multiple overloads at some point.

Interesting.  Future spawn() may spawn the thread in a remote CPU that
doesn't share the memory.  In that case it perhaps helps to limit fun to
be pure, and val to be immutable, as in Erlang.  D already has pure and
transitive const, so the first few steps for the massively-parallel
D programs are already completed! :-)