Class-A surfacing 3/3: blend curves, curved-edge fillets, the G2 corner and continuity analysis (#1642)

Thanks @midasdf.

crates/doc/src/walk.rs

@@ -50,6 +50,7 @@ impl Walk for crate::document::Operation {}
 impl Walk for solidcraft_kernel::Continuity {}
 impl Walk for crate::document::Direction {}
 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::CoilSection {}