Stop exporting lightning::ln::features

Now that the module only contains some implementations of
serialization for the `lightning_types::features` structs, there's
no reason for it to be public.

lightning/src/ln/mod.rs

@@ -21,7 +21,7 @@ pub mod inbound_payment;
 pub mod msgs;
 pub mod peer_handler;
 pub mod chan_utils;
-pub mod features;
+mod features;
 pub mod script;
 pub mod types;