[Feature Request] Improved tracing algorithm in serde_reflection::trace_type

[ma2bd]

🚀 Feature Request

serde_reflection::trace_type relies on a few ideas and heuristics that it might be time to revisit.

Notably, we use the current table of formats tracer.registry to guide the tracing in most decision points -- with the exception of tracer.incomplete_enums that is also used to decide how to trace enum variants.

This approach was a natural starting point but it is not ideal in the long run for a few reasons:

• Calling trace_value before trace_type may update the registry in an incomplete way and defeat further tracing with trace_type.
• We don't have an easy way to detect non-termination if the base case of enums is not the first variant.
• We cannot call trace_type_once<T> repeatedly and hope to make progress in finding variants that are deeper that T. (For instance, after one tracing call on struct T(option<S>) we are not going to look into S ever again because it has a format: https://github.com/novifinancial/serde-reflection/blob/master/serde-reflection/src/de.rs#L199)