Function data_dir_for 

pub fn data_dir_for(id: &str) -> PathBuf

The directory a given app id keeps saved data in, without touching (or creating) it and without going through the process-global published app.

Same platform root and shape as data_dir (<root>/lumen/<id>), but takes the id explicitly: an OS-capability crate that scopes storage to an app id of its own - rather than the current process’s published one - resolves its storage through this, so there is one definition of the shape and not a second copy of it.