`NonEmpty` collections support

[jawbroken]

To quote myself from the other thread:

@jawbroken

I wouldn't say that it is equally useful, given that many methods would require you to drop the non-empty guarantee (e.g. anything that returns a subsequence or can remove elements) and the only upside seems to be that it would remove optionality from a small number of return types (first, popFirst, randomElement, min, max). It's roughly like trying to represent a non-zero number in the type system so you can't divide by zero.

I don't find the analogy to Optional that compelling, because roughly all you can do with an Optional is conditionally unwrap it. That's why I made the analogy to a non-zero number above, i.e. you're going to want to do a lot of Sequence/Collection operations on a non-empty collection but be thwarted or forced to drop the non-empty guarantee.