One walk over a feature's references (2/3, #876) (#988)

Part 2 of #876: one walk over the references a feature holds (the sketches it's built from, the features it names, the bodies it works on) replaces three hand-written matches, with a golden record covering every variant.

crates/doc/src/lib.rs

@@ -19,6 +19,7 @@ mod document;
 mod eval;
 pub mod expr;
 pub mod face_anchor;
+mod feature_refs;
 pub use face_anchor::{FaceAnchor, FaceAttachmentStatus};
 pub mod format;
 mod format_offsets;