Was hardcoded to "[object Object]" for everything; per ES it should
return "[object Array]", "[object Function]", "[object Number]",
etc. by class. Added js-object-tostring-class helper that switches
on type-of and dict-internal markers (__js_*_value__,
__callable__). Prototype-identity checks ensure
Object.prototype.toString.call(Number.prototype) returns
"[object Number]" (similar for String/Boolean/Array).
built-ins/Array: 18/45 → 20/45, built-ins/Number: 43/50 → 44/50.
conformance.sh: 148/148.
giles
8d9ce7838d