msg_iter: make ->each_part method for PublicInbox::MIME
The reliance on Email::MIME->subparts is a tad inefficient with
a work-in-progress module to replace Email::MIME. So move
towards using ->each_part as a class-specific iterator which can
take advantage of more class-specific optimizations in the
yet-to-be-revealed PublicInbox::Eml and PublicInbox::Gmime
classes.
The msg_iter() sub remains for compatibility with existing
3rd-party scripts/modules which use our small public Perl API
and Email::MIME.