Use vis meta-variable is macros

[ratijas]

Declarative macros like the one below could use the relatively new visibility aka :vis macro meta-variable instead of manual pub($tt*) etc. It had some issues in past, but now it's mostly fixed and safe to use.

https://github.com/mystor/rust-cpp/blob/46c1f8d7e6cad764a78ec1e6a421f3f031b5e638/cpp/src/lib.rs#L348-L358