Object.keys/values/entries type definitions should be improved

[Validark]

Currently, the Object.keys/values/entries functions don't have the best types for our purposes. I wrote this up and thought we could use something similar to this in es.d.ts once #66 gets merged.

declare function keys<T>(o: Array<T>): Array<number>;
declare function keys<T>(o: Set<T>): Array<T>;
declare function keys<K, V>(o: Map<K, V>): Array<K>;
declare function keys<T>(o: T): Array<keyof T>;

declare function values<T>(o: Array<T>): Array<T>;
declare function values<T>(o: Set<T>): Array<true>;
declare function values<K, V>(o: Map<K, V>): Array<V>;
declare function values<T>(o: T): Array<T[keyof T]>;

declare function entries<T>(o: Array<T>): Array<[number, T]>;
declare function entries<T>(o: Set<T>): Array<[T, true]>;
declare function entries<K, V>(o: Map<K, V>): Array<[K, V]>;
declare function entries<T>(o: T): Array<[keyof T, T[keyof T]]>;