Factor out Type traversal to a Visit trait
diff --git a/syntax/mod.rs b/syntax/mod.rs
index 87e3454..5422681 100644
--- a/syntax/mod.rs
+++ b/syntax/mod.rs
@@ -27,6 +27,7 @@
 mod toposort;
 pub mod trivial;
 pub mod types;
+mod visit;
 
 use self::attrs::OtherAttrs;
 use self::discriminant::Discriminant;