Thin Extrude: walls of a set thickness along sketch curves (#721) (#991)

Thin Extrude builds walls of a set thickness along sketch curves, open chains and closed loops, with the wall centred, inside, outside or on both sides at two thicknesses. Saved under format 20 (thin on an Extrude); an extrude without thin saves as before. Closes #721.

crates/sketch/src/lib.rs

@@ -33,6 +33,7 @@ mod profiles;
 mod shape;
 mod solver;
 mod text;
+mod thin;
 mod union_find;
 mod wire;
 
@@ -61,6 +62,7 @@ pub use solver::{
     solve_after_edit, solve_after_value_edit, solve_holding, solve_with,
 };
 pub use text::{MAX_TEXT_CHARS, TextAlign, TextStyle, text_geometry, text_geometry_in, text_geometry_styled};
+pub use thin::{MAX_THIN_SEGMENTS, thin_wall_regions};
 pub use wire::{MAX_FIT_POINTS, MAX_WIRE_POINTS, Wire, WireSpace, fit_polyline};
 
 #[cfg(test)]
@@ -73,6 +75,8 @@ mod continuation_tests;
 mod font_tests;
 #[cfg(test)]
 mod tests;
+#[cfg(test)]
+mod thin_tests;
 
 #[cfg(test)]
 mod virtual_corner_tests;