Method swap

include/libfork/core/impl/stack.hpp:240–243

* @brief Swap `lhs` with `rhs. */

   * @brief Swap `lhs` with `rhs.
   */
  inline friend void swap(stack &lhs, stack &rhs) noexcept {
    using std::swap;
    swap(lhs.m_fib, rhs.m_fib);
  }

  /**
   * @brief Destroy the stack object.