[whatwg/fetch] Support Structured Headers in the Headers API (#930)

In particular:

* An algorithm that takes a header name _name_ and a structured header type _type_ and returns the corresponding header's value in some kind of useful object-representation for _type_ (or an error).
* An algorithm that takes a header name _name_ and a structured header value _value_ and replaces all corresponding headers with a new structured header (or errors if _value_ cannot be coerced).

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https://github.com/whatwg/fetch/issues/930