printing of [Spec_cfg] nodes in CFGs

- core.ml now only has type definitions, which are annoying to repeat
  in ml and mli; all operations are in coreOps.ml(i), including pretty
  printing and some conversions
- moved [string_of] from [Debug] to [Corestar_std],
  solving two TODOs from code comments
- added the type ('a pretty_printer) to [Corestar_std]