message: Mark size() const in OperandStack
diff: @@ -99,7 +99,7 @@ public:
  OperandStack& operator=(const OperandStack&) = delete;
  /// The current number of items on the stack (aka stack height).
  - size_t size() noexcept { return static_cast<size_t>(m_top + 1 - bottom()); }
  + size_t size() const noexcept { return static_cast<size_t>(m_top + 1 - bottom()); }
  /// Returns the reference to the top item.
  /// Requires non-empty stack.
  