JSON Decode Exploration

I should really write some elevator-pitch style examples to include here. Oh well.

Made with ❤️ and released under BSD3.

Changelog

Json.Decode.Exploration.Pipeline.optional and optionalAt now error when used on something that is not an object. The clue is that each field in the path is optional in the existential sense - if it simply does not exist, we dutifully use the provided fallback. However, if a field exists but does not point to an object, we now error out instead.
as a side effect, both of the above now correctly mark empty objects as having been used, rather than giving a warning when using optional on an empty object.

Thanks to @michaeljones for surfacing this in #3.

Json.Decode.Exploration.check: verify the value of a field while decoding
Json.Decode.Exploration.Pipeline:
- checked and checkedAt: verify values during decoding
- ignored and ignoredAt: completely ignore values during decoding

Thanks to @michaeljones for the proposal.

Increased test coverage of Json.Decode.Exploration.Pipeline.

BadField errors weren't properly traced through the stack.

Increase test coverage of Json.Decode.Exploration.