Add layoutString to the public API

[ocharles]

It's a shame that if we're building our own pretty printer that wants nice Show instances we have to import from the internal API. What do you think about adding some Doc/SimpleDoc producing functions to the public API?

[georgefst]

Yeah it is a bit silly really, as evidenced by the fact that we use it in our web app. Interested in opening a PR?

[ocharles]

Sure, do you want layoutString and layoutStringAbstract exported?

[georgefst]

I assume layoutString is enough for your use case? I suppose there's an argument that it's too specialised and that the flexibility of layoutStringAbstract is more useful, but we'd need to additionally add haddocks, and probably think of a better name.

[ocharles]

Admittedly I have no actual use ATM, I was just thinking that I might want to pretty print Show types soon, and was considering options.