Implement nested boundaries

[abonander]

This is a feature I've been putting off for a while because I wasn't sure how to approach it. Still not quite sure, but I needed a reminder somewhere so I don't forget it.

[abonander]

Closing as I decided to leave this unimplemented. The complexity it adds doesn't appear to be worth it.

IETF RFC 7578, Appendix A claims:

the method of using a nested "multipart/mixed" from [RFC2388] is no longer recommended for creators and is not required for receivers as there are no known implementations of senders.

(Emphasis mine.)

Prototype implementation is archived on the nested-archive branch for posterity. As of 0.10.0 if a field has Content-Type: multipart/mixed an error will be thrown (now just a note is logged) linking back to this issue.

If anyone comes across any clients that are sending such requests, please feel free to share the details here.