Merge: 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.

Pull-Request: #548
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/json_serialization.nit
Simple merge

diff --cc lib/standard/string.nit
Simple merge