message: Add OperandStack::drop()
diff: @@ -135,6 +135,12 @@ public:
  return *m_top--;
  }
  + void drop(size_t num) noexcept
  + {
  + assert(num <= size());
  + m_top -= num;
  + }
  +
  /// Shrinks the stack to the given new size by dropping items from the top.
  ///
  /// Requires new_size <= size().
  