Implement range indexing

[musjj]

It should be possible to index a NonEmpty with a Range{,From,To,Full}:

fn foo(a: Vec<u8>, b: NonEmpty<u8>) {
    let _ = a[5..];
    let _ = b[5..]; // mismatched types expected type `usize` found struct
                    // `std::ops::RangeFrom<{integer}>`
}