message: Add Stack::drop() method
diff: @@ -15,6 +15,7 @@ public:
  using std::vector<T>::back;
  using std::vector<T>::emplace_back;
  using std::vector<T>::pop_back;
  + using std::vector<T>::resize;
  using std::vector<T>::size;
  void push(T val) { emplace_back(val); }
  @@ -27,5 +28,8 @@ public:
  }
  T peek(size_t depth = 0) const noexcept { return (*this)[size() - depth - 1]; }
  +
  + /// Drops @a num_elements elements from the top of the stack.
  + void drop(size_t num_elements = 1) noexcept { resize(size() - num_elements); }
  };
  } // namespace fizzy
  