1. Special case array. Because array instances are typed as
implementing an interface that lists all their public methods, `keyof
Array` returns all those method names as keys…even though they’re not
actually enumerable at runtime.

2. Intersect keyof T with string, because `keyof T` can return `number
| string` in some cases, whereas the entries’ keys will always be
strings.

Applying the same logic used on Object.entries in the prior commit and
in microsoft#12207

Now that keyof T only returns string or string subtypes (microsoft#12425),
there’s no need to do keyof T & string.