Release Splice version 3.0.0-alpha.21.
Release Splice version 3.0.0-alpha.21.

 * Upgrade `b-tree` to `2.0.0-alpha.52`. Closes #164.
 * Upgrade `riffle` to `2.0.0-alpha.34`. Closes #163.

Closes #165.

bigeasy/splice,bigeasy/splice