Commit creation

[dhardy]

We could redesign PartitionState to store a list or map of changes instead of a
parent sum. clone_child() would then reset the list of changes.

This would optimise for few changes to a large set of elements (which is the
intended usage); the current optimises more for many changes to a small set of
elements.

We could support both of these via separate types, plus possibly a third type
which does not support modification. There would be a clone_... method for
each type and a Partition would need to be able to create a commit from each
type which supports modifications.

• State remembers parent: has to store a Sum (currently 32 bytes)
• State remembers Vec of changes: 24 bytes (I think)
• State remembers HashMap of changes: 40 bytes (I think)
• Code tag #0021

[dhardy]

The main part of this was implemented in order to allow metadata of new states to be set in a "correct" way.

The remaining question: should the new MutPartState continue to operate by mutating a set of elements or instead build a set of changes?