De-share trait_ref

Also, makes the pretty-printer use & instead of @ as much as possible,
which will help with later changes, though in the interim has produced
some... interesting constructs.

src/libsyntax/ast_util.rs

@@ -580,7 +580,7 @@ pub fn view_path_id(p: &view_path) -> node_id {
 
 /// Returns true if the given struct def is tuple-like; i.e. that its fields
 /// are unnamed.
-pub fn struct_def_is_tuple_like(struct_def: @ast::struct_def) -> bool {
+pub fn struct_def_is_tuple_like(struct_def: &ast::struct_def) -> bool {
     struct_def.ctor_id.is_some()
 }