Release B-tree version 2.0.0-alpha.21.
Release B-tree version 2.0.0-alpha.21.

 * Upgrade `turnstile` to `6.0.0-alpha.4`. Closes #609.
 * Upgrade `prospective` to `^0.3.0`. Closes #608.

Closes #610.

bigeasy/strata,bigeasy/strata