higher-level(feat): Handle {De,En}coding of Mailbox

Rust MUAs need to handle {de,en}coding of mailbox (i.e., folder) names on their own. This means, that it might be required to use "decode_utf7", "decode_base64", ... before being able to display a mailbox. Ideally, the higher-level library already does that.

Questions:

* Is there a canonical form?
* Can we use `Cow`?
* What happens if we create a mailbox with colliding encodings?
* -> Not sure, but at least the RFC says "Modified BASE64 MUST NOT be used to represent any printing US-ASCII character which can represent itself." Sounds canonical to me?