Card Table

参考stack上的答案，通俗的讲解了card table是什么，为什么要用card table。

参考链接

https://plumbr.io/handbook/garbage-collection-algorithms-implementations/concurrent-mark-and-sweep

https://www.jianshu.com/p/2a1b2f17d3e4

https://hllvm-group.iteye.com/group/topic/38223#post-248757

https://www.jianshu.com/p/2329d1c43ceb