Realistically, because our btree is protected by CoW checksums, the only
place we can end up with a cycle is in our mroot chain.
This is convenient, as we don't need our btree traversal state when
traversing the mroot chain, so we can put both the tortoise state and
btree traversal state into a union, theoretically saving some RAM.
Unfortunately stack measurements show no change, even though our mtree
traversal in on the hot path. I'm not sure why this is. My best guess is
that the RAM savings is beneath the compilation noise floor, since we
currently only ever create one of these structs.
Christopher Haster
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