Use Map instead of Object as cache#28

so, why not? I use [bench-lru](https://github.com/dominictarr/bench-lru) for benchmarking. before: ``` name, set, get1, update, get2, evict lru-cache, 350, 1449, 546, 2083, 383 hashlru, 6667, 12500, 12500, 12500, 5000 lru, 1852, 4000, 3448, 4348, 13 ``` after: ``` name, set, get1, update, get2, evict lru-cache, 465, 1613, 763, 2222, 364 hashlru, 6667, 14286, 10000, 14286, 5882 lru, 1099, 1923, 2500, 3333, 1124 ``` 🤔