tco.el provides tail-call optimisation for functions in elisp that
call themselves in tail-position.

It works by replacing each self-call with a thunk, and wrapping the
function body in a loop that repeatedly evaluates the thunk.  Roughly
speaking, a function `foo':

(defun-tco foo (...)
  (...)
  (foo (...)))

Is rewritten as follows:

(defun foo (...)
   (cl-flet (foo-thunk (...)
               (...)
               (lambda () (foo-thunk (...))))
     (let ((result-sym (apply foo-thunk (...))))
       (while (is-trampoline-result-p result-sym)
         (setq result-sym (funcall (unwrap result-sym))))
       result-sym)))