Homogenize constructor names.
The pattern-matching construct in the logic is now systematically named "Tcase" in constructors (Ptree.Tmatch -> Tcase). The one in the programs (supporting exceptions) is now systematically named "Ematch" (Expr.Ecase -> Ematch, Dexpr.DEcase -> DEmatch). They are now homogeneous with the other constructors: Term.Tcase, Dterm.DTcase, Ptree.Ematch, Mltree.Ematch. Smart constructor Expr.e_case was renamed accordingly.