Sweep guide rails and two-rail sweep (#771) (#1806)

Thanks @jyviko.

crates/doc/src/walk.rs

@@ -53,6 +53,7 @@ impl Walk for crate::document::FilletFit {}
 impl Walk for crate::document::FilletCorner {}
 impl Walk for crate::document::MeshData {}
 impl Walk for crate::document::RevolveConvention {}
+impl Walk for crate::document::RailFit {}
 impl Walk for crate::document::CoilSection {}
 impl Walk for crate::document::ThinSide {}
 impl Walk for crate::document::ExtendMode {}