Release Twiddle version 2.0.0-alpha.8.
Release Twiddle version 2.0.0-alpha.8.

 * Upgrade `advance` to `3.0.0-alpha.9`.
 * Build with Node.js 12, 14 and 15 on CI/CD.
 * Update `LICENSE` for 2021.

Closes #55.

bigeasy/twiddle,bigeasy/twiddle