Make blueprint code slightly more resilient to changes.

Leverage traits instead of hard-coded type parameters.

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

@@ -109,7 +109,7 @@ where
     }
 }
 
-impl Display for Blueprint<Schema> {
+impl<T: serde::Serialize + Default> Display for Blueprint<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let s = serde_json::to_string_pretty(self).map_err(|_| fmt::Error)?;
         f.write_str(&s)

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)
     }
 }