Release Riffle version 2.0.0-alpha.55.
Release Riffle version 2.0.0-alpha.55.

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

Closes #216.

bigeasy/riffle,bigeasy/riffle