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.