sg14::span_ring: class template argument deduction

[peterlauro]

to benefit from C++17's class template argument deduction the deduction rules on top of span_ring should be considered:

the example of deduction rules

template <typename ContiguousIterator>
    ring_span(ContiguousIterator, ContiguousIterator)->ring_span<typename std::iterator_traits<ContiguousIterator>::value_type>;

    template <typename ContiguousIterator, typename Popper>
    ring_span(ContiguousIterator, ContiguousIterator, Popper)->ring_span<typename std::iterator_traits<ContiguousIterator>::value_type, Popper>;

    template <typename ContiguousIterator>
    ring_span(ContiguousIterator, ContiguousIterator, ContiguousIterator, std::size_t)->ring_span<typename std::iterator_traits<ContiguousIterator>::value_type>;

    template <typename ContiguousIterator, typename Popper>
    ring_span(ContiguousIterator, ContiguousIterator, ContiguousIterator, std::size_t, Popper)->ring_span<typename std::iterator_traits<ContiguousIterator>::value_type, Popper>;