The previous semantics of the `rewind` operation would remove the last
checkpoint, if any, but would not further modify the tree. However,
these semantics are error prone - if you rewind to a checkpoint, you are
not able to rewind to the same checkpoint again; also, in practice, it
doesn't make sense to shift the location of a checkpoint in the note
commitment tree. This change alters `rewind` to (a) take an explicit
checkpoint depth, with depth `0` meaning that any state added since the
last checkpoint should be discarded, and (b) only allow a rewind
operation to succeed if a checkpoint actually exists at the specified
depth.
Kris Nuttycombe
Kris Nuttycombe