message: span: (almost) match std::span's constructor behavior
  c++20's draft of std::span no longer includes move constructors.
diff: @@ -84,6 +84,14 @@ class Span
  C* m_data;
  std::size_t m_size;
  + template <class T>
  + struct is_Span_int : public std::false_type {};
  + template <class T>
  + struct is_Span_int<Span<T>> : public std::true_type {};
  + template <class T>
  + struct is_Span : public is_Span_int<typename std::remove_cv<T>::type>{};
  +
  +
  public:
  constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
  @@ -134,8 +142,19 @@ public:
  * To prevent surprises, only Spans for constant value types are supported when passing in temporaries.
  * Note that this restriction does not exist when converting arrays or other Spans (see above).
  */
  - template <typename V, typename std::enable_if<(std::is_const<C>::value || std::is_lvalue_reference<V>::value) && std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value && std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, int>::type = 0>
  - constexpr Span(V&& v) noexcept : m_data(v.data()), m_size(v.size()) {}
  + template <typename V>
  + constexpr Span(V& other,
  + typename std::enable_if<!is_Span<V>::value &&
  + std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value &&
  + std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
  + : m_data(other.data()), m_size(other.size()){}
  +
  + template <typename V>
  + constexpr Span(const V& other,
  + typename std::enable_if<!is_Span<V>::value &&
  + std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value &&
  + std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
  + : m_data(other.data()), m_size(other.size()){}
  constexpr C* data() const noexcept { return m_data; }
  constexpr C* begin() const noexcept { return m_data; }
  