Release Splice version 3.0.0-alpha.12.
Release Splice version 3.0.0-alpha.12.

 * Upgrade `b-tree` to `2.0.0-alpha.37`. Closes #135.
 * Upgrade `riffle` to `2.0.0-alpha.22`. Closes #134.

Closes #136.

bigeasy/splice,bigeasy/splice