Add an `Empty` tuple vector like the `Only` vector.

While empty vectors exist in `Data.Vector.Fixed.Cont` as `empty`, this
seems like an obvious addition in line with the single tuple vector
`Only`.
Exported `Empty` from `Data.Vector.Fixed`.
Added `mk0` and `N0` constructors and types.

To complement the existence of these empty types it's meaningful to
extend some of the helper functions and types.
Merge pull request #3 from tel/dev-empty-vec

Add an `Empty` tuple vector like the `Only` vector.