* lisp/emacs-lisp/cl-macs.el: Optimize self-calls in tail position
Implement a limited form of tail-call optimization for the special
case of recursive functions defined with `cl-labels`. Only self-recursion
is optimized, no attempt is made to handle more complex cases such a mutual
recursion.
The main benefit is to reduce the use of the stack, tho in my limited
tests, this can also improve performance (about half of the way to
a hand-written `while` loop).
(cl--self-tco): New function.
(cl-labels): Use it.