Use `Object.is` instead of `===` to implement phys_equal

[TyOverby]

There are a few places where the behavior of phys_equal differ between native OCaml and Jsoo:

a	b	native	jsoo
NaN	NaN	true	false
0.0	-0.0	false	true

The (relatively) new Object.is function implements those comparisons "correctly":
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is