Join walls swept along splines in 2D (#908)

Walls swept along a spline have flat B-spline tops and bottoms, so booleans between them went to the general intersection, which fails on coincident spline faces. Such faces are now put on true planes, and two prisms of the same height and direction are decided in 2D with their spline outlines fitted by arcs within a micron. The result is kept only when its volume is plausible; otherwise the general boolean takes over. The free-form flag is cleared by a drop guard so an unwinding boolean can't leave it set.

Fixes #844.

crates/kernel/src/lib.rs

@@ -59,6 +59,9 @@ mod par;
 pub use par::{for_each_on_threads, map_on_threads};
 #[cfg(test)]
 mod pinch_tests;
+mod planar_faces;
+#[cfg(test)]
+mod planar_faces_tests;
 mod plane_fit;
 mod polybool;
 mod polyhedron;