code: func (c *LRU) Keys() []string {
  	keys := make([]string, len(c.items))
  	i := 0
  	for ent := c.evictList.Back(); ent != nil; ent = ent.Prev() {
  		keys[i] = ent.Value.(*entry).key
  		i++
  	}
  	return keys
  }
docs: // Keys returns a slice of the keys in the cache, from oldest to newest.