message: [crispy] span: Adds at(size_t) accessors.
diff: @@ -52,6 +52,20 @@ class span {
  constexpr reference_type front() const noexcept { return begin_[0]; }
  constexpr reference_type back() const noexcept { return std::prev(end_); }
  + reference_type at(size_t i)
  + {
  + if (i >= size())
  + throw std::invalid_argument("i");
  + return begin_[i];
  + }
  +
  + const_reference_type at(size_t i) const
  + {
  + if (i >= size())
  + throw std::invalid_argument("i");
  + return begin_[i];
  + }
  +
  private:
  pointer_type begin_;
  pointer_type end_;
  