Remove dependency on Transformers

We were only using this for Data.Functor.Identity; in addition, we'd
been declaring an orphaned Show instance for that, which is gross.
Better just to define our own Identity. (Close #18)

Also, since Vinyl now ships with two of them, I've moved Validation and
Identity into Data.Vinyl.Idioms.*.
Fix Intro.lhs
Merge pull request #34 from VinylRecords/dependency-cleanup

Remove dependency on Transformers