WIP: Visitor pattern implementation for serialization

[garrett-is-a-swann]

Reworks/merges the JSON/Script serialization functions to use a single entity-component "graph" traversal mechanism. This mechanism can also be used by user code in order to perform arbitrary serializations, and/or as a map/reduce mechanism.

[SanderMertens]

I like the idea of a visitor that understands the reflection data and provides a simpler user-facing API. The current reflection API is complex, and is also not documented very well.

However, I think such a visitor API would be a better fit for C++ as there the compiler is able to optimize out virtual calls if it knows the type of the visitor. In C having this many callbacks hurts performance since they can't be inlined. Both the JSON serializer and flecs script are performance critical, so I don't think this PR is moving things in the right direction.

Some thoughts on improving the usability of the reflection API:

• A reflection manual is sorely needed
• Examples that show how to iterate the TypeSerializer
• I'd like to explore whether the flecs::cursor API can be extended to provide easier iteration of members/elements
• A C++ visitor-based approach could start out as an external repository
• If extending existing APIs proves too cumbersome, the visitor could get upstreamed to flecs