Release Memento version 0.1.43.
Release Memento version 0.1.43.

 * Upgrade `b-tree` to `2.0.0-alpha.116`.
 * Upgrade `amalgamate` to `0.1.74`.
 * Upgrade `ascension` to `^3.0.0-alpha.7`.

Closes #143.

bigeasy/memento