Internally, store message buffers as bytes

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
copy content from actor buffer to RayPlatform buffer

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
don't repeat the MPI datatype

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
use MPI_BYTE for MPI calls (instead of MPI_UNSIGNED_LONG_LONG)

Link: https://github.com/sebhtml/RayPlatform/issues/2
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
enable payloads that are not multiple of 8 bytes

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
run until all actors died

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>