experimental distance of terms dropped

in particular, there is no more need to define intermediate
sorts or to close the set of protected types wrt subtyping

You must run ./config.status after this commit

- provide a method to mark proof attempts as obsolete
  (thus, we can replay a saved tree even if the source
  file hasn't changed)
- cleaning applies to a subtree (like transformations
  and provers), not only at the first level
- obsolete proof attempts do not count as successuful

also apply transformations to the proved subgoals if "all subgoals"
is selected (just as we do it for provers)

- create_env_of_loadpath is now provided in Env instead of Lexer
- find_channel functions now depend on format to determine the
  suitable extensions