JSON Decode Exploration

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

Made with ❤️ and released under BSD3. My eternal thanks to @michaeljones for his invaluable feedback and continued discovery of edge-cases and bugs!

Changelog

Json.Decode.Exploration:
- isObject and isArray: ascertainments about structure without using contained values

Json.Decode.Exploration.Pipeline:
- optional and optionalAt now use isObject to prevent marking unused contained values as having been used.

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.

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

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

BadField errors weren't properly traced through the stack.

Increase test coverage of Json.Decode.Exploration.