Add Iterators to object and array

[hughfdjackson]

As closely as possible, provide an Iterator interface for im.object and im.array. Including:

• Iteration over one element in a collection of n shouldn't incur O(n) penalty (i.e. no full mutable)
• Iterating over arrays should go from 0 -> .length - 1 inclusive
• Iterating over objects should iterate over all keys
• Where StopIteration is available, use it; where not, throw an error

Reference: http://wiki.ecmascript.org/doku.php?id=harmony:iterators