Upgrade `b-tree` to `2.0.0-alpha.64`.
Upgrade `b-tree` to `2.0.0-alpha.64`.

Strata 2.0.0-alpha.64 upgrades Recriprocate to 0.0.2 which we did
here in the last commit.

https://github.com/bigeasy/strata/releases/tag/v2.0.0-alpha.64
https://github.com/bigeasy/reciprocate/releases/tag/v0.0.2

Closes #190.

bigeasy/splice,bigeasy/splice