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This closes issue #122

Bench fixed

This commit might break the bench, I will fix it quickly I promise :)

Bench reestablished

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.

both constructor and projection names are now added to the mod_known from
Mltree.pmodule, pointing to the type declarations in which they are introduced.

- range types that fit in 31-bit signed integers are mapped to OCaml's type int
  (both literals and for loops)
- a for loop on a type that is mapped to 'int' in the OCaml driver is
  translated to an OCaml for loop

If [e1] is a ghost expression but [if e1 then e2 else e3] doesn't, then either
[e2] or [e3] (or both) are Eabsurd.

If [e] is a ghost expression but [match e with bl] doesn't, then the first
pattern in [bl] is irrefutable.