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

Thanks @jijinbei.

crates/engine/src/cmd/construct.rs

@@ -135,8 +135,8 @@ fn geo(s: &Session, v: Option<&Value>, cmd: &str, what: &str) -> Result<GeoRef>
     Ok(g)
 }
 
-fn t_param(s: &Session, p: &Value, cmd: &str) -> Result<String> {
-    let t = if p.get("t").is_some() { req_expr(cmd, p, "t")? } else { "0.5".to_string() };
+fn t_param(s: &Session, p: &Value, cmd: &str) -> Result<solidcraft_doc::Expr> {
+    let t = if p.get("t").is_some() { req_expr(cmd, p, "t")? } else { "0.5".into() };
     check_expr(s, &t, solidcraft_doc::expr::Kind::Unitless, cmd, "t")?;
     Ok(t)
 }