First working draft of thread-local binding.

First working draft of thread-local binding.

At least 2 of the 9 kinds of variable bindings should work ok --
defvaralias variables and ordinary variables.
All other kinds will exhibit weird behavior.
A simple program that works:

   (defvar zz t)
   (defvar buffer-a (get-buffer-create "zardoz"))

   (defun tha ()
     (dolist (v '("A" "B" "C" "D"))
	(message "thread a: %s" v)
	(yield))
     (setq zz nil))

   (defun thb ()
     (dolist (v '("E" "F" "G" "H" "I"))
	(message "thread b: %s" v)
	(yield)
	))

   (with-current-buffer buffer-a
     (run-in-thread 'tha))
   (thb)

   (while zz
     (yield))