Prepare for follow-on change

crates/fj-core/src/core.rs

@@ -15,14 +15,16 @@ pub struct Core {
 impl Core {
     /// Construct an instance of `Core`
     pub fn new() -> Self {
-        Self {
-            layers: Layers::default(),
-        }
+        Self::from_layers(Layers::default())
     }
 
     /// Construct an instance of `Core`, using the provided configuration
     pub fn with_validation_config(config: ValidationConfig) -> Self {
         let layers = Layers::with_validation_config(config);
+        Self::from_layers(layers)
+    }
+
+    fn from_layers(layers: Layers) -> Self {
         Self { layers }
     }
 }