clippy: whitelist inapplicable lints

Author: apoelstra

clippy.toml

@@ -1 +1,3 @@
 msrv = "1.48.0"
+# plan API returns Self as an error type for an large-ish enum
+large-error-threshold = 256

src/plan.rs

@@ -702,6 +702,7 @@ impl Assets {
     pub fn new() -> Self { Self::default() }
 
     /// Add some assets
+    #[allow(clippy::should_implement_trait)] // looks like the `ops::Add` trait
     pub fn add<A: IntoAssets>(mut self, asset: A) -> Self {
         self.append(asset.into_assets());
         self