codingPath more often than not actually needs to be copied, not just
referenced. This makes a big difference for nested containers and
subobjects, which were getting the wrong codingPath values when asking
for them.
This also adds unit tests for JSONEncoder and PropertyListEncoder to
confirm expected behavior.
* Integrate {JSON,PropertyList}{Encoder,Decoder} types to facilitate
encoding types in JSON and property list formats
* Adds Foundation-specific extensions to allow errors exposed from the
stdlib to bridge to NSErrors
