message: Fix return type of span::cbegin and friends
diff: @@ -51,7 +51,7 @@ public:
  using iterator = pointer;
  - using const_iterator = const pointer;
  + using const_iterator = const_pointer;
  using reverse_iterator = std::reverse_iterator<iterator>;
  @@ -94,7 +94,7 @@ public:
  return begin_;
  }
  - constexpr iterator cbegin() const noexcept {
  + constexpr const_iterator cbegin() const noexcept {
  return begin_;
  }
  @@ -102,24 +102,24 @@ public:
  return begin() + size_;
  }
  - constexpr iterator cend() const noexcept {
  - return begin() + size_;
  + constexpr const_iterator cend() const noexcept {
  + return cbegin() + size_;
  }
  constexpr reverse_iterator rbegin() const noexcept {
  return reverse_iterator{end()};
  }
  - constexpr reverse_iterator crbegin() const noexcept {
  - return reverse_iterator{end()};
  + constexpr const_reverse_iterator crbegin() const noexcept {
  + return const_reverse_iterator{end()};
  }
  constexpr reverse_iterator rend() const noexcept {
  return reverse_iterator{begin()};
  }
  - constexpr reverse_iterator crend() const noexcept {
  - return reverse_iterator{begin()};
  + constexpr const_reverse_iterator crend() const noexcept {
  + return const_reverse_iterator{begin()};
  }
  // -- element access ---------------------------------------------------------
  