message: [crispy] span: adds new ctor (iterator, size_t)
diff: @@ -21,6 +21,7 @@ class span {
  // constructors
  //
  constexpr span(iterator _begin, iterator _end) noexcept : begin_{_begin}, end_{_end} {}
  + constexpr span(iterator _begin, size_t _count) noexcept : begin_{_begin}, end_{std::next(_begin, _count)} {}
  constexpr span() noexcept : begin_{}, end_{} {}
  constexpr span(span<T> const&) noexcept = default;
  constexpr span(span<T>&&) noexcept = default;
  @@ -49,6 +50,9 @@ class span {
  pointer_type end_;
  };
  +template <typename T>
  +span(T, size_t) -> span<decltype(std::declval<T>().begin())>;
  +
  template <typename T> constexpr bool operator==(span<T> const& a, span<T> const& b) noexcept
  {
  if (a.size() != b.size())
  