Message Passing Interface (MPI) in Nit and partial C language module

mpi.nit brings parallel programming to Nit using Message Passing Interface with OpenMPI. Will be used to implement a distributed tester.

c.nit offers low-level structures for performance critical code. More structure and services could be added as needed.

Warning: Does not yet pass all tests on nitg-e, will need to solve the "No provided declaration" error on extern classes with virtual types. I'll update this PR to solve this soon.
Je sais pas trop, c'est autant facile de se tromper coté Nit que coté C.
Passer par des simili Int et des méthodes de conversion me parait etre une mauvaise solution.
Si l'objectif est d’améliorer l'API C, autant le faire de façon plus propre, non?