Constrain v2.0.0-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.