Continuation labels and variable identifiers may be integers

Continuation labels and variable identifiers may be integers

* module/language/cps.scm (label-counter, var-counter): New parameters,
  for producing fresh label and var names.
  (fresh-label, fresh-var): New procedures.
  (let-fresh): New macro, will replace let-gensyms.
  (build-cps-term): Use let-fresh.

* module/language/tree-il/compile-cps.scm: Use let-fresh to generate
  fresh names.

* module/system/vm/assembler.scm (make-meta, begin-kw-arity): Allow
  exact integers as labels.
  (link-debug): Explicitly mark low-pc as being an "addr" value.