Release Amalgamate version 0.1.21.
Release Amalgamate version 0.1.21.

 * Upgrade `splice` to `3.0.0-alpha.19`. Closes #128.
 * Upgrade `riffle` to `2.0.0-alpha.32`. Closes #127.

Closes #129.

bigeasy/amalgamate,bigeasy/amalgamate