notmuch show: Include output for the enclosing multipart part of a MIME mail

Previously, the outer multipart part of any multipart/mixed,
multipart/signed, etc. MIME message was silently omitted from the
"notmuch show" output. This prevented any client from correctly
determining to which parts a signature applies, for example.

Now, we actually emit these parts as their own parts. The output is
still flattened---the contained parts are not yet included "within"
the multipart part---so it's still not possible to determine to which
parts a signature applies, but this is one step along the path.

The test suite is updated to reflect this change, (though we'll
eventually want to fix the emacs interface to not display buttons for
the multipart enclosure parts as there's nothing useful for the user
to actually do with them).