Subtyping test with perfect hashing in the Nit vm

This pull request contains the first files for the Nit virtual machine.

In this PR, the vm constructs runtime structures for each MClass of the model. These structures represent virtual tables for classes. When constructed, the virtual tables only reserve memory for method pointer but the method call is made with the naive_interpreter mechanisms.   

The virtual machine can now performs subtyping test with the perfect hashing approach.

In the perfect_hashing module, numbering of items now start at 1 instead of 0.


C"est bien ce que je pensais, en termes de doc et d'API, ça va faire louche l'apparition de `ph` dans NaiveInterpreter