Support immutable containers

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YAML.framework does not currently support kYAMLReadOptionImmutable except for NSString, this change uses the optimized ::copy message of NSMutableArray and NSMutableDictionary to make container immutable after the population of the graph. ::copy returns self in an immutable container. Also to support this the YAML.framework API is changed to always return immutable types whether or not the actual reference points to a mutable type or not. If you want a mutable container, cast it, if you use the flag kYAMLReadOptionImmutable casting will fail.