Code should use unidirectional channels, where appropriate

[ThomasHabets]

The API docs would be easier to read if e.g. Client.Fetch took a write-only channel. Then it would be clear that it's an output.

[emersion]

Indeed. Unfortunately this doesn't bode well with our (arguably bad) implementation in responses.Fetch, which is used for both the client and the server.

[emersion]

Closing because go-imap v2 stops using Go channels in the public API.