experiment: eagerly parse structured headers

[frasertweedale]

Currently we parse all headers as unstructured and defer further parsing (e.g. address lists)
to the accessor optics for particular headers.

This issue proposes to experiment with a redesign:

• eagerly parse structured headers defined in the RFC(s) according to their grammars
• represent these directly in the Message data type and enforce occurrence rules and relationships with other fields (see https://datatracker.ietf.org/doc/html/rfc5322#section-3.6)
• update accessors to read these fields
• evaluate the ergonomics of the change

If it proves to be a worthwhile change, make a plan to land the change and write a migration plan.