Revisit open Task questions

[duesee]

• Use Type, impl, and impl Trait ordering
  • I usually use Type, Implementations, Trait Implementations (similar to the Rust docs) in imap-types and imap-codec. In imap-flow it's not always consistent. I think this could be our default. But, in the end, let's use what makes most sense for a specific situation :-)
• Use builder for Tasks
• Fetch vs Fetch{Single,Many}
• clone()s required due to Task trait
• Consistency between seq and uid
• Discuss ...Data data structures
• ...

Some guidance about implementing Tasks (potentially useful for docs) ...

• Don't drop usable information (see functions-expose-intermediate-results-to-avoid-duplicate-work-c-intermediate)
  • If another representation makes more sense, provide an explicit conversion
• Remember most recent Data
  • Replace duplicate Data emitting a warn!ing (using structured values)
  • Replace Data when there is a code with the same meaning in Status emitting a warn!ing (using structured values)
• ...