Trivial instance of Text.Shakespeare.Text.ToText

I’ve been using a trivial orphan instance of `ToText` in some of my code to simplify a few things.  `ToText` is for turning things into `Builder`s, so it makes sense to `instance ToText Builder` with the identity function.

I believe this shouldn’t break anything except for others who may be defining the same instance as an orphan, but we get warnings for that anyways.