Support for async functions

It would be useful to transpile async functions, they're a valid part of the language.

See here for previous discussion: https://gitlab.com/Rich-Harris/buble/issues/71

The `fast-async` package provides a leaner/faster result than regenerator (let's not use regenerator), and I think it can be further optimized because when I convert async functions to Promise-based equivalents manually, they always come out with the leanest result. The new `Promise.finally` will help with this.

This may require a Promise polyfill to be supplied by the user, and that should be documented.

Start by reading the previous discussion in GitLab issue 71 and comparing the proposed fast-async approach with the requested Promise-based output. Determine the transpilation scope, Promise polyfill expectations, and optimization criteria before defining tests and an implementation plan.

Written by the indexing model from the issue text.