Release Splice version 3.0.0-alpha.40.
Release Splice version 3.0.0-alpha.40.

 * Use Operation handle cache.
 * Upgrade `turnstile` to `6.0.0-alpha.52`.
 * Upgrade `b-tree` to `2.0.0-alpha.80`.
 * Upgrade `riffle` to `2.0.0-alpha.55`.

Closes #209.

bigeasy/splice,bigeasy/splice