Release Constrain version 2.0.0-alpha.4.
Release Constrain version 2.0.0-alpha.4.

 * Upgrade `advance` to `3.0.0-alpha.10`.
 * Upgrade `mvcc` to `2.0.0-alpha.4`.
 * Build with Node.js 16 on Travis CI.
 * Build with Node.js 16 on GitHub Actions.

Closes #70.

bigeasy/constrain,bigeasy/constrain