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 (and we get
warnings for that anyways).