Extract SMTP implementation to a separate crate

[amousset]

We currently expose a low-level SMTP interface in lettre. It may make more sense to move it to a separate crate and remove it from lettre public API.

If so, we could re-use the smtp crate name.

[paolobarbolini]

Sounds like an idea we should definitely consider. Not only for SMTP but also maybe for the email builder. Idk sometimes it feels like a single crate is doing too much. Let's think more about it in the coming months.

[malobre]

This would be great, hyper (an http library) went through a similar process where they put protocol specific types in the http crate.