Untrim: holes removed and a face restored to its natural bounds (#1150)

Adds surface.untrim (#761); the SurfaceUntrim feature is first saved in format 21.

crates/kernel/src/step_in/mod.rs

@@ -11,7 +11,7 @@ mod brep;
 pub(crate) mod geom;
 pub(crate) mod p21;
 mod parallel;
-mod split;
+pub(crate) mod split;
 
 use std::collections::{BTreeMap, HashMap, HashSet};
 use std::sync::Arc;