Make blueprint code slightly more resilient to changes.

Leverage traits instead of hard-coded type parameters.

crates/aiken-project/src/blueprint/schema.rs

@@ -334,9 +334,10 @@ impl Data {
     }
 }
 
+// Needed because of Blueprint's default, but actually never used.
 impl Default for Schema {
     fn default() -> Self {
-        Schema::Unit
+        Schema::Data(Data::Opaque)
     }
 }