This branch contains my attempt to enable the following with the exchangers:
Directly using per-rank basic MPI datatypes (and count) with the buffers (and an added displacement), such that
the packing exchangers can be modified to use the basic exchangers instead of hard-coded point-to-point message passing routines,
make the exchangers usable in cases where the MPI datatypes of the built-in redists simply have too high overhead.
extend exchangers to use MPI_Ibsend, MPI_Issend, MPI_Bsend, or MPI_Ssend, where applicable.