Typescript types?

Its a bit difficult to write a definition file... any ideas how to handle that?

Would be easier if the export was the factory function that returns the exported functions:

```js
export default function worker() {
function expensive(time) {
let start = Date.now(), count = 0
while (Date.now() - start < time) count++
return count
}
return {expensive}
}
```

then the types would be identical if the functions return promises.