types: define NonEmpty type constructor

[davidchambers]

I'll explain how I arrived at this pull request.

1. I wrote something like this, forgetting to specify the types:

   const f = def('f', {}, [], _f);

2. I ran the code and got an unhelpful error message:

   RangeError: Invalid array length

3. I set out to add a length check and a custom error message for an empty type array.

4. I remembered that we have an elegant way to avoid ad hoc type checking. :D

5. I defined NonEmptyArray for internal use to improve the error message.

6. I decided to export NonEmptyArray (and NonEmptyString, for symmetry).

[safareli]

I think you can define one NonEmpty(Type) for any Type which is Eq and Monoid eq(a, empty(a.constructor))

[davidchambers]

Wonderful suggestion, Irakli! Please have a look at the updated patch. :)