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/lib.rs

@@ -71,6 +71,8 @@ pub enum DocError {
 
 pub type Result<T> = std::result::Result<T, DocError>;
 
+#[cfg(test)]
+mod feature_walk_tests;
 #[cfg(test)]
 mod named_projection_tests;
 #[cfg(test)]