Collapse `tracing` package into `cats.effect` to avoid bloating the FFI

[djspiewak]

The fact that tracing is a separate package means that TracingConstants needs to be public. We can avoid that problem if we squeeze it into the cats.effect package, along with the other implementation details.

[vasilmkd]

We now cannot use class TracingConstants in the unsafe package. This is frustrating.

[vasilmkd]

At this points, it's either public class or code duplication.

[armanbilge]

Or, becoming a flat-earther 😆