Make an internal API warning macro

[chebbyChefNEQ]

We often expose internal APIs. These APIs are pub but really shouldn't be used as we will not garueentee the stability.

Maybe we could use depercated mechanism to generate a warning when someone touches these APIs.

Maybe there is a better way, this ticket is to track and solve this problem.

[chebbyChefNEQ]

potentially interesting solution: https://github.com/rust-lang/api-guidelines/discussions/95 (via feature flag)