Replace all let-gensyms uses with let-fresh

Replace all let-gensyms uses with let-fresh

* .dir-locals.el: Add with-fresh-name-state.
* module/language/cps.scm (fresh-label, fresh-var): Signal an error if
  the counters are not initialized.
  (with-fresh-name-state): New macro.
  (make-cont-folder): New macro, generates an n-ary folder.
  (compute-max-label-and-var): New function, uses make-cont-folder.
  (fold-conts): Use make-cont-folder.
  (let-gensyms): Remove.

* module/language/cps/arities.scm:
* module/language/cps/closure-conversion.scm:
* module/language/cps/constructors.scm:
* module/language/cps/dce.scm:
* module/language/cps/elide-values.scm:
* module/language/cps/reify-primitives.scm:
* module/language/cps/specialize-primcalls.scm: Use let-fresh instead of
  let-gensyms, and wrap in a with-fresh-name-state as needed.

* module/language/tree-il/compile-cps.scm: Remove hack to avoid
  importing let-gensyms from (language tree-il).