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_ide_api/src/completion/presentation.rs

@@ -1,5 +1,5 @@
 //! This modules takes care of rendering various defenitions as completion items.
-use hir::{Docs, HasSource, HirDisplay, PerNs, Resolution, Ty};
+use hir::{Docs, HasSource, HirDisplay, PerNs, Resolution, Ty, TypeWalk};
 use join_to_string::join;
 use ra_syntax::ast::NameOwner;
 use test_utils::tested_by;