Rename Ty -> Type

Rust source code does not abbreviate type -> ty (the way it abbreviates impl and
mod, for example). This commit updates the naming in syn to reflect that.
diff --git a/tests/test_precedence.rs b/tests/test_precedence.rs
index 87dada8..19dc3ad 100644
--- a/tests/test_precedence.rs
+++ b/tests/test_precedence.rs
@@ -345,7 +345,7 @@
             pat
         }
 
-        fn fold_ty(&mut self, ty: Ty) -> Ty {
+        fn fold_type(&mut self, ty: Type) -> Type {
             ty
         }
     }