[w3c/webcomponents] [idea] Allow HTML Element attributes to accept any type of value, not just strings. (#519)

#517 covers two things: 

1. ability to define attributes for custom elements (and optionally specify getters/setters for those attributes)
2. Ability for DOM API to accept anything, not just strings, for attributes.

I'd like #517 to be about the first idea, and would like to make this issue specifically about the ability for DOM APIs like `setAttribute`/`getAttribute` to be able to accept any JavaScript values besides strings so that we can avoid performance costs from serialization/deserialization. We would be able to pass objects, arrays, numbers, instances of custom classes, etc.

Please see #517 for details (maybe I can move those details here...).

For example, imagine some error component specialized in displaying errors, who's error attribute can accept an actual instance of an `Error`:

```js
try {
    // ...
}
catch (err) {
    let el = document.createElement('error-display')
    el.setAttribute('error', err)
    // ...
}
```

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