Non-string dictionary keys #3

Currently Preserve errors out when it encounters a dictionary key that's not a string. Need to see if it's viable to support other keys.

I think we want to maintain the current rule where output dicts always have string keys. To be able to handle other objects as keys on the Python side would require the class_key escaped dict extension we've been considering.

In that scenario, we'd create a class_key escaped dict (where rather than just a class name as the value, we create a whole new dict) beside the keys that were still strings. In the escaped dict we'd then have key pairs something like 'key_1', 'val_1', 'key_2', 'val_2', which then held the preserved content of the key and value pair from the original raw dict. We have to pull the values over into the escaped section too - as leaving them in the non-escaped section would risk a naming collision (we'd have to name them with a string key of some sort).