Why does `Gearbox.Ecto.transition_changeset/3` return a tuple?

[jtdowney]

I wanted to start a discussion before sending a PR. First, I want to say thanks for releasing this. I was trying to find a state machine library to add to a new project and was finding small things I didn't like with the current landscape until I saw gearbox. One minor thing that surprised me was that Gearbox.Ecto.transition_changeset/3 returned a tuple that is either {:ok, changeset} or {:error, changeset}. This differs from what I expected because changesets already contain their error discriminants with valid? and errors. Right now, I am just piping the result to elem(1), so it is only a minor annoyance, but I wanted to open a discussion on a proposed PR to return the changeset without the tuple wrapper.