Release Constrain version 2.0.0-alpha.0.
Release Constrain version 2.0.0-alpha.0.

 * Convert to ES6, no starting boundary check. Closes #49. Closes #63.
 * Add chicklets and links to `README.md`. Closes #62.
 * Build with Actions, ship coverage to Codecov. Closes #60. Closes #61.
 * Add `.npmignore`. See #62.
 * Update `.gitignore`. See #62.
 * Remove `t` directory. See #62.
 * Build with only Node.js 12 on Travis CI. Closes #59. Closes #58.
 * Upgrade `proof` to 9.0.2. Closes #57.
 * Update `LICENSE` for 2020. Closes #56.

Closes #64.

bigeasy/constrain,bigeasy/constrain