refactor: hide pgp module from public API

src/keyring.rs

@@ -44,10 +44,6 @@ where
         self.keys.push(key);
     }
 
-    pub fn len(&self) -> usize {
-        self.keys.len()
-    }
-
     pub fn is_empty(&self) -> bool {
         self.keys.is_empty()
     }