Migrate autofill to async/await

[Firehed]

The initial callback-based API was made on some incorrect assumptions on how async/await works in a javascript context. Instead, the autofill API should also use promises.

Depending on naming, this could be a breaking change - although it'd also be possible to do this in a compatible way. Since this is pre-1.0, this isn't necessarily a huge concern; all else being equal I'd prefer to have only a single code path to maintain and support.

Note that there are examples on the website and documentation for autofill using the callback design, so they should also get updated to match.