Change signature of Object.keys to return (keyof T)[]

This seems like a simple thing, although I know there's probably a reason it's not already like this.

Currently the signature of `Object.keys` is:
```ts
{
  keys(o: object): string[];
}
```

But why not make it more specific?:
```ts
{
  keys<T extends object>(o: T): (keyof T)[];
}
```