Support Structured Field Values in the Headers API

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).

Considerations:

• Should append() continue to function? It probably should (destroying the type in the process, except perhaps if it's a list or dictionary), otherwise converting existing headers to be structured headers would be difficult.
• How should the getter operation operate on a structured header? Should it pretend to serialize first and then parse to retain maximum compatibility? E.g., being able to interpret something as a list and an item?

The issue text is the only stated entry point; start by reviewing the Fetch Standard's Headers API and the requested structured-field algorithms. Resolve the open questions about append(), getter compatibility, coercion, replacement, and errors; done requires agreed behavior for each operation.