Draft: Attempt at fuller exchanger interface

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.