Generate patches with local pure moves

[thsutton]

Implement support for pure move operations (i.e. where the moved value is only moved, not modified, after application of the patch) between sibling pointers.

1. Track the value deleted by a delete operation.
2. Pair add and delete operations affecting sibling pointers with matching values.
3. Fix up operations that target sibling keys and occur after either of the paired operations in the patch.

This will support the very simplest moves operations (#52) but is very far from complete move support.