Clean src

Since there is groups in the c_src compiler, use them in src to regroupe related concerns.

Thus, this PC create 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 to discuss:

* 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. It this a good idea?
* what to do with the remaining modules in the src directory?

~~~
annotation.nit
astbuilder.nit
astprinter.nit
astutil.nit
astvalidation.nit
c_tools.nit
highlight.nit
literal.nit
location.nit
markdown.nit
modelbuilder.nit
model_utils.nit
model_viz.nit
neo.nit
parser_util.nit
phase.nit
platform.nit
rapid_type_analysis.nit
toolcontext.nit
transform.nit
version.nit
vm.nit
~~~