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/api.rs

@@ -1,4 +1,4 @@
-pub mod appservice_server;
-pub mod client_server;
-pub mod ruma_wrapper;
-pub mod server_server;
+pub(crate) mod appservice_server;
+pub(crate) mod client_server;
+pub(crate) mod ruma_wrapper;
+pub(crate) mod server_server;