Thin Extrude: walls of a set thickness along sketch curves (#721) (#991)

Thin Extrude builds walls of a set thickness along sketch curves, open chains and closed loops, with the wall centred, inside, outside or on both sides at two thicknesses. Saved under format 20 (thin on an Extrude); an extrude without thin saves as before. Closes #721.

apps/solidcraft-cli/src/recipe.rs

@@ -296,6 +296,17 @@ pub fn to_script(recipe: &Value) -> Result<Value, String> {
             Some("sketch") => sketch_cmds(f, &mut out)?,
             Some("extrude") => {
                 let mut p = json!({"sketch": profile_sketch(f), "profiles": profiles_of(f), "operation": op(f), "name": name, "body_names": bodies});
+                // Thin: {sketch, curves, thickness, side?, thickness2?} instead of profiles.
+                if let (Some(thin), Some(params)) = (f.get("thin"), p.as_object_mut()) {
+                    params.remove("profiles");
+                    params.insert("thin".into(), json!(true));
+                    params.insert("sketch".into(), thin.get("sketch").cloned().unwrap_or(Value::Null));
+                    for key in ["curves", "thickness", "side", "thickness2"] {
+                        if let Some(v) = thin.get(key) {
+                            params.insert(key.into(), v.clone());
+                        }
+                    }
+                }
                 extent(f, &mut p);
                 out.push(json!({"command": "solid.extrude", "params": p}));
             }