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

thus we gain more goals than we lose

also preserve proposition names in split_premise when possible

What was its purpose in the first place? Integers are protected
in Simplify anyway and then we can simply forget the difference
between the infinite sorts (as we do in encoding_tptp).

until we understand why keeping them makes nightlies regress

This permits using them in transformations, as they also
preserve the term structure.

Export Term.t_ty_map and Term.t_app_map. We can generalize
t_s_map to cover t_app_map, but this requires computing the
list of types for every application, which we rarely need.

which gives us a cheap test for recursive defitions.
We could also make an effort to ignore the conclusions
in inductive predicate definitions but it's probably
not worth it.

(sorry for not doing it earlier)

also, export Ty.ty_v_* traversal functions