Feature walks: generate them from field roles with #[derive(Walk)] (#1251)

The three walks over a feature (expressions, references, geometry) are generated from FeatureKind by #[derive(Walk)] in a new proc-macro crate, solidcraft-derive, from #[walk(...)] attributes written next to each field. The hand-written walks in params.rs, feature_refs.rs and feature_geometry.rs are gone; Ruled Surface and Extend have their attributes, and the three golden walk records are unchanged.

Co-authored-by: jijinbei <[email protected]>

xtask/src/layers.rs

@@ -36,6 +36,8 @@ impl Class {
 /// prefix.
 pub const TABLE: &[(&str, Class)] = &[
     ("geom", Class::Layer(0)),
+    // the derive macros (proc-macro, no workspace dependencies)
+    ("derive", Class::Standalone),
     ("sketch", Class::Layer(1)),
     ("kernel", Class::Layer(1)),
     ("render", Class::Layer(1)),