message: freelist: use std::swap
diff: #define MESH__FREELIST_H
  #include <random>
  +#include <utility>
  #include "rng/mwc.h"
  @@ -73,11 +74,7 @@ public:
  swapOff = mwc.inRange(_off, maxCount() - 1);
  }
  - const fl_off_t a = _list[_off];
  - const fl_off_t b = _list[swapOff];
  -
  - _list[_off] = b;
  - _list[swapOff] = a;
  + std::swap(_list[_off], _list[swapOff]);
  }
  inline size_t pop() {
  