Root higher-order CPS term is always $kfun $cont

* module/language/cps/arities.scm:
* module/language/cps/closure-conversion.scm:
* module/language/cps/compile-bytecode.scm:
* module/language/cps/constructors.scm:
* module/language/cps/contification.scm:
* module/language/cps/cse.scm:
* module/language/cps/dce.scm:
* module/language/cps/elide-values.scm:
* module/language/cps/prune-bailouts.scm:
* module/language/cps/prune-top-level-scopes.scm:
* module/language/cps/renumber.scm:
* module/language/cps/self-references.scm:
* module/language/cps/simplify.scm:
* module/language/cps/specialize-primcalls.scm:
* module/language/tree-il/compile-cps.scm: Adapt to produce and consume
  raw $kfun $cont instances.

* .dir-locals.el: Update $letrec indentation.

.dir-locals.el

@@ -27,6 +27,7 @@
      (eval . (put '$continue           'scheme-indent-function 2))
      (eval . (put '$kargs              'scheme-indent-function 2))
      (eval . (put '$kfun               'scheme-indent-function 4))
+     (eval . (put '$letrec             'scheme-indent-function 3))
      (eval . (put '$kclause            'scheme-indent-function 1))
      (eval . (put '$fun                'scheme-indent-function 1))))
  (emacs-lisp-mode . ((indent-tabs-mode . nil)))