converted to new naming scheme and use MPI_Count

we are going to follow the MPI standard pattern of using MPI_Foo_x for
the MPI_Count version of MPI_Foo, hence all BigMPI functions are now
named MPIX_Foo_x (because MPI_ is a reserved namespace) and we use
MPI_Count instead of bigmpi_int_t.
refactored everything in the MPIX_Foo_x form