fix-letrec tweaks

fix-letrec tweaks

* module/language/tree-il/fix-letrec.scm (partition-vars): Previously,
  for letrec* we treated all unreferenced vars as complex, because of
  orderings of effects that could arise in their definitions.  But we
  can actually keep simple and lambda vars as unreferenced, as their
  initializers cannot cause side effects.
  (fix-letrec!): Remove letrec* -> letrec code, as it's unneeded.