removed module Transform
big refactoring: split Term.t into FOTerm.t (first-order) and HOTerm.t (higher-order),
therefore split substs, formulas, type inference, unification and substitutions to deal with at least one of
the two types.
module Lambda for lambda calculus with HOTerm;
modules HO and FO that gather submodules for an easier naming;
Transform is back (prev commit was a mistake)