Refactor message, send_msg(), sign()

[RoDmitry]

• New send_msg(&mut self, message: &Message<'x>) to send message which was already transformed using IntoMessage. Now you can have owned Message, so if any error occurs, you don't loose the entire message, so you can send it again.
• New sign(&mut self, signer: &DkimSigner<_>) method on Message, which signs message in place, and then message can be sent using usual send().
• Couple #[inline], because compiler does not inline by default, as far as I know, so you need to tell it which function you think may be inlined. It improves performance.
• Sized traits

[RoDmitry]

Feel free to give feedback in case you’re interested in merging only part of these changes.

[mdecimus]

Feel free to give feedback in case you’re interested in merging only part of these changes.

Hi, I won't have time to review this PR until Stalwart Mail Server reaches v1.0.0.