- Introduced optional callback for Deno.core.serialize API, that returns
cloning error if there is one.
- Removed try/catch in seralize structured clone function and throw error from
callback.
- Removed "Object with a getter that throws" assertion from WPT.
Currently all async ops are polled lazily, which means that op
initialization code is postponed until control is yielded to the event
loop. This has some weird consequences, e.g.
```js
let listener = Deno.listen(...);
let conn_promise = listener.accept();
listener.close();
// `BadResource` is thrown. A reasonable error would be `Interrupted`.
let conn = await conn_promise;
```
JavaScript promises are expected to be eagerly evaluated. This patch
makes ops actually do that.
