Turnstile v6.0.0-alpha.0

• Add chicklets to README.md. Closes #156.
• Don't need a decorator. Closes #144.
• Externalize work queue termination. Closes #155.
• Upgrade avenue to 0.2.0-alpha.3. Closes #154.
• Upgrade destructible to 5.2.0-alpha.5. Closes #153.
• Upgrade interrupt to 9.0.0-alpha.4. Closes #152.
• Implement a drain watch method. Closes #151.
• Best-foot-forward rejector. Closes #150.
• Convert Check to use async/await. Closes #143.
• Convert Set to use async/await. Closes #142.
• Remove dead code. See #141.
• Nested method in enqueue needs to be async. See #141.
• enqueue retruns Promise, should not be async. See #141.
• onvert Queue to use async/await. Closes #141.
• Build with Actions, ship coverage to Codecov. Closes #149.
• Install Proof at Travis CI. See #145.
• And body to Named argument to entry function. Closes #148.
• Convert enter to named arguments. Closes #147.
• Replace destroy with drain. Closes #146.
• Convert tests back to Proof. Closes #139. Closes #145.
• Install Mocha at Travis CI. See #139.
• Sketch of Decorator. See #144.
• Build with Node.js 12 on Travis CI. Closes #140.
• Convert Turnstile to ES6. See #139. Closes #138.

Closes #157.