Releasing thread and URL reference when the workerized function is not needed

Great library, very useful stuff and absolutely love the size. :)

I've only recently started learning about `Web Workers` and took a look at the source code. So apologies in advance if I am wrong ;). 2 things caught my eyes:

```js
const workerURL = URL.createObjectURL(new Blob([script]));
// Create an "inline" worker (1:1 at definition time)
const worker = new Worker(workerURL);
```

So if we do something like the following snippet (taken from the README), it seems that each new function instantiated via `greenlet(...)` will reserve a new thread and a new URL reference.

```js
import greenlet from 'greenlet'

let getName = greenlet( async username => {
let url = `https://api.github.com/users/${username}`
let res = await fetch(url)
let profile = await res.json()
return profile.name
})

console.log(await getName('developit'))
```

So, if there is a case wherein I don't need to use `getName` after a certain point in my code, those resources are still trapped. They may be very less in size to be of a practical concern, but I am not sure about it and would love if anyone can comment on that.

However, if the output function `getName` comes with a `dispose` method which releases those references, it could be useful. WDYT? Something like:

```js
getName.dispose() // Release references
```

Internally, it could call:

```js
window.URL.revokeObjectURL(workerURL);
worker.terminate();
```

Post `dispose`, `getName` can itself become `undefined` so it's not callable. Or can `throw` a more informative error: `The function is disposed/discarded due to .dispose() call.`.

Is there a downside to this approach if the contributors already considered any similar approach?