enable unreachable_pub lint

This causes some other lints to start firing too (which is good), but
I'm going to fix them in follow-up commits to keep things organized.

src/service/appservice/data.rs

@@ -2,7 +2,7 @@ use ruma::api::appservice::Registration;
 
 use crate::Result;
 
-pub trait Data: Send + Sync {
+pub(crate) trait Data: Send + Sync {
     /// Registers an appservice and returns the ID to the caller
     fn register_appservice(&self, yaml: Registration) -> Result<String>;