Align `PackedDataChecksum::process` code path with `save` code path

• in PackedDataChecksum, re-use the save infrastructure to make sure that by construction the checksum is computed on exactly the same data that is actually saved as it now uses the same implementation
• remove the as_tuple infrastructure which is no longer required

note: sits on top of !3244 (closed) which fixes a lifetime issue in as_tuple -- so where !3244 (closed) fixes a bug, this eliminates the code in question...