active rma/reductions; fix minor error

move RMA and reductions into uncommented region since they have already
been or will soon be implemented.

changed result_count from int to MPI_Count
add RMA impl

apparently i forgot to git add this before.
reorganize header with comments

note implementation challenges and priority in head.
move reductions that are being supported into active code.
add implementation of blocking, non-v reductions

reduce and allreduce use chunking.
reduce_scatter_block uses reduce+scatter but might need some polish.