create cats.data.NonEmptyVector

[ceedubs]

We seem to have arrived on a decision in #537 that NonEmptyVector should have its own data type. Let's go for it!

Since representing a Vector as case class NonEmptyVector[A](head: A, tail: Vector[A]) is not very efficient for quite a few operations, @non has suggested that we create a class that wraps a Vector that it knows to be non-empty. We can guarantee this by making the companion object method that creates an instance of the class from a Vector return an Option.

[zainab-ali]

I'm working on this in #1137

[ceedubs]

Resolved by #1137