new register allocator and calling convention

[Stephen Weeks]

> So  Thread.copyCurrent  is  the  equivalent  of fork (but in the thread world
> instead of the process world)?  No, that can't be right.  I'm confused  about
> Thread.saved/Thread.clearSaved.

Thread.copyCurrent puts a copy of the current thread in a magic place
("savedThread").  Thread.saved returns what is in the magic place (and
NULLs it) and Thread.clearSaved NULLs it.

> This  implementation  still  does, it looks like, a full copy of the stack at
> callcc time and also when you call the continuation, right?

Yes.  But not when you just switch threads.  I.E. one shot
continuations are constant time.