This adds most of the thread features visible to emacs lisp.

This adds most of the thread features visible to emacs lisp.

I roughly followed the Bordeaux threads API:

http://trac.common-lisp.net/bordeaux-threads/wiki/ApiDocumentation

... but not identically.  In particular I chose not to implement
interrupt-thread or destroy-thread, but instead a thread-signalling
approach.

I'm still undecided about *default-special-bindings* (which I did not
implement).  I think it would be more emacs-like to capture the let
bindings at make-thread time, but IIRC Stefan didn't like this idea
the first time around.

There are one or two semantics issues pointed out in the patch where I
could use some advice.