Clean src

Since there is groups in the c_src compiler, use them in src to regroupe related concerns.

Thus, this PR creates new groups:

* modelize (AST-> model)
* semantize (AST+model -> augmented AST)
* frontend (modelize+semantize+other stuff)
* compiler (all compiler-related things)
* interpreter (all interpreter related things)

Some more things remain to discuss latter:

* the status of platforms things: I moved them to compiler, does this make sense?
* I let the nit*.nit and test_*.nit at the top level to have a fast glance of what are the entry points/main modules. Is this a good idea?
* what to do with the remaining modules in the src directory?