For the benefit of unkeyed containers, coding paths currently contain optional `CodingKey`s — unkeyed containers are allowed to report a `nil` key in a path. However, this is unhelpful for debugging purposes, or inspecting the coding path for context, since any unkeyed container simply reports `nil`, whether it’s at index 1 or 1000.
Now all containers are required to report a non-optional CodingKey for their segment of the coding path, and coding paths are now exposed as `[CodingKey]`.
CustomNSError briding only works when the CustomNSError conformance is in the same module as the original error declaration. We need to sink these down into the standard library.
* 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
