Three related fixes:
1. Every JS function body binds arguments to (cons p1 ... __extra_args__),
so arguments[k] and arguments.length work as expected.
2. Array.from(iter, mapFn) invokes mapFn through js-call-with-this
with the index as second arg (was (map-fn x), missing index and
inheriting outer this).
3. thisArg defaults to js-global-this when omitted (per non-strict ES).
conformance.sh: 148/148.
giles
f0dffd275d