Implement caf_sendget_by_ref

[vehre]

Implement caf_sendget_by_ref, which can be done in an initial version as a combination of caf_get_by_ref and caf_send_by_ref.

Here also gcc/libgfortran/caf/single.c::_caf_sendget_by_ref may serve as a template. May be using it as is might also do the trick in the beginning.

#289 is about optimizing the transfer.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[vehre]

Fixed indirectly with #792