macros: `scoped_thread_local` should be private (#2470)
macros: `scoped_thread_local` should be private (#2470)

Do not export the `scoped_thread_local` macro outside of the Tokio
crate. This is not considered a breaking change as the macro never
worked if used from outside of the crate due to the generated code
referencing crate-private types.
tokio-rs/tokio,tokio-rs/tokio