<rdar://problem/13434476>

Making value objects properly iterable in constructs of the form
[ x for x in value_with_children ]

This would previously cause an endless loop because lacking a proper iterator object, Python will keep calling __getitem__() with increasing values of the index until it gets an IndexError
since SBValue::GetValueForExpressionPath() supports synthetic array members, no array index will ever really cause an IndexError to be raised, hence the endless iteration

class value_iter is an implementation of __iter__() that provides a terminating iterator over a value