Notify when an LRU item is removed

[davidmartos96]

It would be useful to have some way of knowing when an item is removed from the LRU. If there is some cleanup/disposing logic that need to be done to the stored values, right now there is no way to do it when using a maximumSize, as the entries are removed implicitly.

I propose the following API

final lruMap = LruMap(
    onItemRemoved: (key, value) {
         value.dispose();
    },
)

[AlessandroToschi]

Nice, I was going to implement the same feature!
Having a callback when an item is evicted from the cache would be really useful!