Move from ZeroCopy to the rental crate

[jonhoo]

The imap::ZeroCopy struct is effectively an implementation of the "rental pattern", which already has a good crate implementation. It would be good to switch to the rental crate, but it would also constitute a breaking change, as rental creates a separate struct for each wrapped type.

[jonhoo]

Alternatively I wonder if we could use the zerocopy crate.

[avitex]

How about the owning_ref crate? The rental crate author recommends moving away from his, and the zerocopy crate is in google source

[jonhoo]

I think owning_ref might be harder to fit this use-case to if I remember correctly from last time I used it, but by all means, if you can get it to work, that'd be awesome!

[avitex]

The owner of owning_ref has kindly given control to me, so I can make a few changes.
I ran into a problem myself when I tried to use it, but should be able to resolve it soon enough :)

[dario23]

async-imap, a fork of this repo, has introduced rental in their code, maybe we could apply some of the same refactorings here?