Conform to the Rust API guidelines

While `0.9` made some significant improvements to the `imap` crates' interface, we still have some way to go. In particular, we should go through the [Rust API guidelines checklist](https://rust-lang-nursery.github.io/api-guidelines/checklist.html) and do the best we can to match its recommendations. I think this is probably a blocker for 1.0.

See the original issue here: https://github.com/mattnenterprise/rust-imap/issues/100

Start by reading the Rust API guidelines checklist and comparing its recommendations with the public interfaces of the imap crates. Record which recommendations apply and identify the affected APIs; done would require addressing the applicable gaps and verifying that the crates conform consistently.

Written by the indexing model from the issue text.