Adjust `Realm` visibility

[raskad]

Currently Realm is a public struct and it's only public method is crate(). The only reason for this seems to be, so that we can benchmark the creation of a realm.

Either

1. Realm is fully private and we need to change how we benchmark it, or
2. we should provide some functionality for the public Realm e.g; changing the Realm of a Context. But is that api usefully in any way?

[wusyong]

For the entry point of DOM types, I think we also have to set a Window instance to be realm's global object. https://html.spec.whatwg.org/multipage/window-object.html#script-settings-for-window-objects

[jedel1043]

Resolved by #2801