One walk over a feature's expressions (1/3, #876) (#902)

inputs_mut, inputs and expressions are now built on one exhaustive walk (walk_exprs / walk_exprs_mut) instead of two hand-written matches over FeatureKind. inputs() no longer clones the feature, plane chains are walked with the same nesting cap for inputs as for expressions, and a new variant without a walk arm does not compile. A golden record of every variant's expressions guards the walk.

crates/doc/src/construct.rs

@@ -575,15 +575,6 @@ impl GeoRef {
     }
 }
 
-impl GeoRef {
-    fn plane_ref(&self) -> Option<&PlaneRef> {
-        match self {
-            GeoRef::Plane { plane } => Some(plane),
-            _ => None,
-        }
-    }
-}
-
 impl AxisDef {
     pub fn refs(&self) -> Vec<&GeoRef> {
         match self {
@@ -594,11 +585,6 @@ impl AxisDef {
         }
     }
 
-    /// Planes it refers to (whose expressions it depends on).
-    pub fn planes(&self) -> Vec<&PlaneRef> {
-        self.refs().into_iter().filter_map(GeoRef::plane_ref).collect()
-    }
-
     pub fn refs_mut(&mut self) -> Vec<&mut GeoRef> {
         match self {
             AxisDef::ThroughFace { face } => vec![face],
@@ -621,10 +607,6 @@ impl PointDef {
         }
     }
 
-    pub fn planes(&self) -> Vec<&PlaneRef> {
-        self.refs().into_iter().filter_map(GeoRef::plane_ref).collect()
-    }
-
     pub fn refs_mut(&mut self) -> Vec<&mut GeoRef> {
         match self {
             PointDef::At { point } => vec![point],