Prepack: Calling .toString() on an abstract number or string should be supported

[NTillmann]

As it's a safe operation, making this test case work:

(function() {
  let x = global.__abstract ? __abstract("number", 42) : 42;
  let s = x.toString();
  let t = s.toString();
  global.inspect = function() {
    return t;
  }
})();

This should address #613.

[wdhorton]

I can tackle this one

[jeffreytan81]

@wdhorton, are you still working on it? If not, I would like to take over. Thanks.