msg_iter: new internal API for iterating through MIME
Unlike Email::MIME::walk_parts, this is non-recursive and gives
depth + index offset information about the part for creating
links for later retrieval
It is intended for read-only access and changes are not
propagated to the parent; however future versions of it
may clobber bodies or the original version as it iterates
to reduce memory overhead.
It is intended for making it easy to locate attachments within a
message in the WWW view.