Make type walking infrastructure a bit nicer

If/when we switch to using Chalk's Ty, we'll need to replace this by its `Fold`
trait, but I didn't want to import the whole thing just yet.

crates/ra_hir/src/lib.rs

@@ -69,7 +69,9 @@ pub use self::{
     resolve::Resolution,
     source_binder::{PathResolution, ScopeEntryWithSyntax, SourceAnalyzer},
     source_id::{AstIdMap, ErasedFileAstId},
-    ty::{display::HirDisplay, ApplicationTy, CallableDef, Substs, TraitRef, Ty, TypeCtor},
+    ty::{
+        display::HirDisplay, ApplicationTy, CallableDef, Substs, TraitRef, Ty, TypeCtor, TypeWalk,
+    },
     type_ref::Mutability,
 };