Provide a better API for writing custom Output impls

[soenkehahn]

Currently users of cradle are not meant to write impls for their own types for the Output trait. It's in theory possible, but it's neither designed for that nor documented. This issue is meant for tracking progress on solving that.

This currently has a very low priority, since I haven't stumbled upon a very good use-case for this. And without a use-case, it doesn't seem like a good idea to try to come up with a design or solution. But if you

• are using cradle for something, and
• find yourself in a situation where it's hard or impossible to do something easily, and
• think this could be solved with an Output impl for a custom type,

then please comment here.

(See also #65 for some previous discussion.)