WIP: Add information to use mail-parser in an IMAP server

[superboum]

It seems that my previous patch was not totally correct (for example, it does not work with DecodeResult::Borrowed).
So I have rewritten it to to use only stream.pos and state.mime_boundary.as_ref().
But I am not totally satisfied yet: the specific problem is when there is a RFC822 message contained in an RFC5322 message, mail-parser wrongly include the MIME delimiter from the parent in the child, RFC822 email. I want to fix it and validate all this logic on your test emails.

Retrospectively, I am not sure that offset_last_part is really needed, but having a body_raw in Part is required: an IMAP server must not decode content and return its raw size, not its decoded size.

Also, I am a bit worried with my new fields: it might break your tests, so we need to check that and maybe update them.
I will not be available in the next weeks but hope to finish this work until the end of the summer.

In the end, I think you should take into account this information if you want to make a release during this time: my previous PR might not be that great. If you really need to make a release, I would recommend that you rollback it.

Sorry, I thought that the problem might be easier to address.

[mdecimus]

Unfortunately at the moment I do not have the time now to review this PR or implement this feature. Could you for the time being use these changes directly from your forked repository?
Another possibility is that your IMAP server re-encodes in Base64 all message parts instead of the returning the raw parts. But I am not sure how strict are the compliance tests run by Dovecot's imaptest.

[mdecimus]

@superboum I had some time today and I was able to make these changes. Now each part contains the raw offsets for the header and body, you can obtain them as follows:

let (start, mid, end) = part.raw_offsets();

The headers would be raw_message[start..mid] while the contents are accessible using raw_message[mid..end].

Please could you check if they pass the Dovecot tests?