RemoveListener support closure function; fix: #9

[RelicOfTesla]

fix https://github.com/gookit/event/issues/9

[RelicOfTesla]

还有个简易方法，使用reflect.Value做比较。 即

func getListenCompareKey(src Listener) reflect.Value {
    return reflect.ValueOf(src)
}

缺点就是不支持传入reflect.Struct的Listener，必须是指针型的Listener或func。也就是必须On(...)时，对传入的Listener进行判断，panic掉struct的Listener，强制要求&xxx{}或new或ListenerFunc类型进来

func (em *Manager) addListenerItem(name string, li *ListenerItem) {
    if name != Wildcard {
        name = goodName(name)
    }
    if li.Listener == nil {
        panic("event: the event '" + name + "' listener cannot be empty")
    }
    if reflect.ValueOf(li.Listener).Kind() == reflect.Struct {
        panic("must use pointer Listener, don't use struct Listener")
    }

    // exists, append it.
    if lq, ok := em.listeners[name]; ok {
        lq.Push(li)
    } else { // first add.
        em.listenedNames[name] = 1
        em.listeners[name] = (&ListenerQueue{}).Push(li)
    }
}

我另外push一个过来

[inhere]

@RelicOfTesla 感谢PR 👍