some refactors to make the code more readable

[JayceFayne]

The goal of this PR was to parse embedded rfc822 messages (eagerly).
For example:

From: [email protected]
To: [email protected]
Subject: Wrapper message
MIME-Version: 1.0
Content-Type: message/rfc822

From: [email protected]
To: [email protected]
Subject: Embedded message
MIME-Version: 1.0
Content-Type: text/plain

Hi

Is not being parsed but instead you get a binary body...which is fine.
It turns out adding this feature would just add unneccesarry complexity. I am opening this PR to upstream some refactors that make the parse_mail_recursive function much more readable.

[staktrace]

Thanks! Overall this is a nice cleanup. I'd prefer to drop the second commit but the other three look good

[JayceFayne]

Dropped the second commit

[staktrace]

Please rebase and resolve conflicts here. Thanks again for all the PRs!