add ES3 fallbacks for Array.isArray and Object.keys

The [MDN polyfill][1] is unappealing:

```javascript
// From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
if (!Object.keys) {
  Object.keys = (function() {
    'use strict';
    var hasOwnProperty = Object.prototype.hasOwnProperty,
        hasDontEnumBug = !({ toString: null }).propertyIsEnumerable('toString'),
        dontEnums = [
          'toString',
          'toLocaleString',
          'valueOf',
          'hasOwnProperty',
          'isPrototypeOf',
          'propertyIsEnumerable',
          'constructor'
        ],
        dontEnumsLength = dontEnums.length;

    return function(obj) {
      if (typeof obj !== 'object' && (typeof obj !== 'function' || obj === null)) {
        throw new TypeError('Object.keys called on non-object');
      }

      var result = [], prop, i;

      for (prop in obj) {
        if (hasOwnProperty.call(obj, prop)) {
          result.push(prop);
        }
      }

      if (hasDontEnumBug) {
        for (i = 0; i < dontEnumsLength; i++) {
          if (hasOwnProperty.call(obj, dontEnums[i])) {
            result.push(dontEnums[i]);
          }
        }
      }
      return result;
    };
  }());
}
```

I opted for a simpler version which doesn't make allowances for a bug in old versions of Internet Explorer. I'm happy to switch to the foolproof (but ugly!) fallback if desired.

Also, this is interesting:

> In ES5, if the argument to this method is not an object (a primitive), then it will cause a `TypeError`. In ES6, a non-object argument will be coerced to an object.
>
> ```javascript
> > Object.keys("foo")
> TypeError: "foo" is not an object // ES5 code
>
> > Object.keys("foo")
> ["0", "1", "2"]
> ```

What's the desired behaviour in our case? We should avoid `Object.keys` entirely to avoid generating code with environment-dependent semantics.


[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys