Implement the Map interface

[alexgleason]

I want to pass lru-cache into my custom class, which accepts any Map.

lru-cache and ttl-cache work perfectly in runtime, but I have to cast them using cache as Map<string, T>.

I added implements Map<K, V> to this class, and tweaked it to match. All it needed was a few type assertions, and the addition of [Symbol.toStringTag]. No functionality is changed.

[alexgleason]

@isaacs Bump

[isaacs]

Shipped on 10.2, thanks!

[alexgleason]

@isaacs Thank you!