Upgrade `b-tree` to `2.0.0-alpha.66`.
Upgrade `b-tree` to `2.0.0-alpha.66`.

`b-tree` was updated for an update to its `ascension` dependency which is not
used here. See the `b-tree` change log for details on the changes to
`ascension`.

https://github.com/bigeasy/b-tree/releases/tag/v2.0.0-alpha.66

Closes #256.

bigeasy/locket,bigeasy/locket