The structure of the `incrementalmerkletree` and `bridgetree` types has
not historically been stable, and as such it is not appropriate to
automatically derive serialization for these types.
This change exposed an inconsistency in how `BridgeTree` and
`CompleteTree` were tracking `Mark` operations, which has now also been
fixed.
The behavior of the "unmark" operation has been simplified, such that it
now ensures that a marked node will be made eligible for garbage
collection when the checkpoint for the tree state in which the mark was
forgotten rolls off the back of the checkpoint queue.
Kris Nuttycombe