[runtime] Immediately set the array-index hash in Uint32ToString and convert to string in ForInPrepare

This speeds up for/in over arrays. E.g.,:

function f(a) {
  for (let i in a) {
    if (a[i] != a[i]) print(false);
  }
}
var a = new Array(10000000);
a.fill(1);
f(a);

runs 3x faster after the change.

BUG=chromium:703226

Change-Id: Iabc5e931d985a03f89440cd702b2feb3eb9f5c18
Reviewed-on: https://chromium-review.googlesource.com/459538Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44108}