Switched to Strict Text everywhere

My gut feeling is that is still a bit rough around the edges, but it's a good groundwork. Probably we need to decide how and if do the folding, because previously we were using a lazy `Builder` whilst now we have just a monolithic `Text` string we `mappend` all the time.

I've also polished a bit the code, using hlint's suggestions.

My patch though *doesn't solve* issue #35