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

As noted in the previous commit message, `b-tree` was updated because
it's `ascension` dependency has changed. You can look at the `b-tree`
change log for the details of those changes. They do not affect this
module.

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

Closes #199.

bigeasy/splice,bigeasy/splice