lfu-cache:恒定时间LFU缓存的生锈实现

高速缓存 大致基于的恒定时间最少使用（LFU）高速缓存的简单实现。 例子 use lfu_cache :: LfuCache; let mut cache = LfuCache :: with_capacity ( 2 ); // Fill up the cache. cache. insert ( "foo" , 3 ); cache. insert ( "bar" , 4 ); // Insert returns the evicted value, if a value was evicted, in case additional // bookkeeping is necessary for the value to be dropped. let maybe_evicted = cache. insert ( "baz" , 5 ); // In the case of a t