Extrude: a symmetric distance is the total thickness (#1260)

A symmetric solid extrude of 10 mm is now 10 mm thick, 5 mm each side of the sketch plane, and switching from One Side to Symmetric keeps the thickness entered. Under format 21 the extent records its convention; designs saved without it keep the per-side reading, so their geometry and parameters don't change. Fixes #1236.

Co-authored-by: Karl-VonRichter <Karl-VonRichter@users.noreply.github.com>

crates/doc/src/walk.rs

@@ -53,6 +53,7 @@ impl Walk for crate::document::ThinSide {}
 impl Walk for crate::document::ExtendMode {}
 impl Walk for crate::document::RuledMode {}
 impl Walk for crate::document::RuledCorners {}
+impl Walk for crate::document::ExtrudeConvention {}
 
 /// Containers walk what they hold, in order.
 macro_rules! walk_each {