Generalize JSON macro to any serializer

I really like json! macro but would want to use it even with other serializers or at least for streaming serialization of JSON structure (not constructing entire Value tree in-memory and then flushing altogether).

It looks like it should be possible to add intermediate macro that would accept $serializer:ident and use it with serialize_map / serialize_seq instead of creating values, and then json! macro would be just a wrapper that calls intermediate one with internal Serializer just like to_value currently does, but I've noticed the comment about breaking changes and wondered if this is something that can be considered or should be done outside of this project.

cc @SergioBenitez

Start by reading the existing json! macro and the to_value entry point, then examine how serialize_map and serialize_seq relate to the proposed serializer input. Done would require a settled API direction for generalized or streaming serialization, followed by implementation and validation; the issue names no files or tests.