Change the form of the language so that a term is always ended by a variable.

- Change the Star form in the raw syntax and in the term;
- Add Star in the definition;
- The "normal form" is now a couple (Env,Ident);
- A type can be either a variable or a sort, this is done to avoid bootstraping issues.
Make undefineds a bit more clear
Add a mapfold function for env passing functions.
Fix sort stuffs and implement checkDec.
Fix lookup issue, was previsouly considering position in the map.

Improved error handling in the same shot.
Enhance unification. It should work on sort at least.
My first example!