[whatwg/fetch] Add remaining domintro boxes (#1161)

For the Headers class, Body mixin, Request's clone(), and the Response class.

Closes #543.

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I would very much appreciate review in the form of fixup commits as this is extremely tedious work. 😊 I didn't really try to tackle any greater style questions, though I would be (more) inclined to address suggestions there as I realize that could use some renewal, especially now that JavaScript has destructuring.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1161


-- Commit Summary --

  * Add remaining domintro boxes

-- File Changes --

    M fetch.bs (132)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1161.patch

https://github.com/whatwg/fetch/pull/1161.diff


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