code: func (pool *SyncPool) Free(mem []byte) {
  	if size := cap(mem); size <= pool.maxSize {
  		for i := 0; i < len(pool.classesSize); i++ {
  			if pool.classesSize[i] >= size {
  				pool.classes[i].Put(&mem)
  				return
  			}
  		}
  	}
  }
docs: // Free release a []byte that alloc from Pool.Alloc.