Tighten up [Serializable] and [Transferable] definitions

[domenic]

• Prohibits use on interfaces involving inheritance, since we weren't
  handling that.
• Removes the attempt at supporting mixins. Mixins have changed, so the
  text was no longer correct, and it was never very complete anyway.

This should serve as a better foundation for #4940.

---

/structured-data.html ( diff )

[annevk]

File has [Serializable]. I think the way to maybe do this is to align with how File API already tackles this. Each interface can have this, but each interface is then also responsible for the full algorithm. And if you have a lot of inheritance you can sort out deduplication yourself, until that becomes common enough that abstraction is warranted. (Currently File API duplicates, as it's fairly simple, which seems fine.)

[domenic]

Thank you for catching that. Pushed a new commit that does that. New commit message body suggestion:

* Spells out how when interitance gets involved, specs need to handle
  the whole object; we don't do any automatic
  inheritance-chain-crawling.
* Removes the attempt at supporting mixins. Mixins have changed, so the
  text was no longer correct, and it was never very complete anyway.

[annevk]

Also works well for https://drafts.fxtf.org/geometry-1/#structured-serialization too.