* opt/split.sml: seriously reworked.  Now splits TFNs as well.
* kernel/primop.sml (effect): removed DEREF and MAKEREF from the pure
  primops.
* flint/flintutil.sml (copy): sort the type-map since {lt,tc}_subst
  actually expects it to be sorted.
* kernel/ltyextern.{sml,sig} (lt_nvpoly): new function (digged from
  recover.sml to build a LT_POLY in the case of named tvars.
* opt/recover.sml: removed unused depth arguments left behind from before
  we switched to named tvars.
(addLty): added for the benefit of opt/split.sml
(lt_nvpoly): moved to ltyextern.sml