Feature fields: a type for each role (Expr, BodyName, FacePick…) (#1273)

Thanks @jijinbei.

crates/engine/src/occurrences.rs

@@ -258,7 +258,7 @@ impl Session {
         let mut removed = std::collections::BTreeSet::new();
         for feature in self.doc.features.iter().skip(marker).filter(|f| !f.suppressed) {
             if let FeatureKind::Remove { bodies } = &feature.kind {
-                removed.extend(bodies.iter().filter(|b| !kept.contains(b.as_str()) && shown.body(b).is_some()).cloned());
+                removed.extend(bodies.iter().filter(|b| !kept.contains(b.0.as_str()) && shown.body(&b.0).is_some()).map(|b| b.0.clone()));
             }
         }
         if removed.is_empty() || self.doc.occurrences.is_empty() {