Parse only needed fields

[Raimo33]

Is your feature request related to a problem? Please describe.
My feature aims to significantly improve deserialization speed when expected keys are known beforehand

Describe the solution you'd like
Make it possible to initialize the deserializer so that only the wanted fields (precomputed at compile time) are extracted and considered in the parsing phase.

Additional context
This technique is not widely used, and I believe it could set yyjson even further from competition

[ibireme]

Right now, yyjson's API is DOM-based. If you know the fields ahead of time, use yyjson_obj_iter_get() for better performance.

But to skip unneeded content, we need a new set of APIs.

[Raimo33]

Has this been considered in the past?

[ibireme]

This was discussed before (e.g. #199 (comment)), but doing it in a clean and usable way with the C API seems quite complex.