Sequester v2.0.0-alpha.0

• Convert to async/await. Closes #69.
• Sketch.
• Caret pin Proof. See #67.
• Fix broken Proof invocation. See #67.
• Add badges and links to README.md. Closes #94.
• Update LICENSE for 2020. Closes #68.
• Build with Actions, ship coverage to Codecov. Closes #65. Closes #66.
• Upgrade proof to 9.0.2. Closes #67.
• Update .gitignore. See #64.

Closes #71.