Add MPI_Info_* support.

This adds MPI_Info_create, MPI_Info_free, MPI_Info_get, MPI_Info_dup, MPI_Info_delete, MPI_Info_get_nkeys, MPI_Info_get_nthkey, MPI_Info_get_valuelen functions.
This is just basically a xbt dict wrapper.

We may have issues with the get_nthkey, as the xbt dict does not really keep the order.. but this mpi call is hum ... stupid? aniway.