Doc output instead of String

[cdsmith]

I'd like for the output to be a Doc (from either https://hackage.haskell.org/package/pretty or a similar package) instead of a String. This would:

1. Be completely convertible to String when needed, so no loss of functionality.
2. Make it easier to generate nice output from users of the type.
3. Make it easier to compose explanations from combinators by nesting bulleted lists and the like.

Point 3 is a really critical one. Right now, the output from nesting combinators is hard to read and parse, and this would go a long way toward fixing it.