Non empty lists

[adamnfish]

It'd be nice to try and embed more of the business rules in the types to make implementations simpler. In particular, it's impossible for there to be 0 collections on a front, so that could be represented with a non-empty-list type. This would allow us to safely call head to get the first collection without having to provide dummy fallback content (see the upcoming feature on "canonical collections").

Scalaz has a NonEmptyList type that would suit this very well.

Not urgent, but it'd make the code clearer and adding some new features easier!