Refactor the self-info so that the def-id is carried in ty_self()

and the fn_ctxt doesn't need any self_info field at all.

Step towards fixing `fn(&self)` (cc #4846) to have a distinct
lifetime.

src/libsyntax/ast.rs

@@ -185,7 +185,7 @@ pub enum def {
                       /* trait */  Option<def_id>,
                       purity),
     def_self(node_id, bool /* is_implicit */),
-    def_self_ty(node_id),
+    def_self_ty(/* trait id */ node_id),
     def_mod(def_id),
     def_foreign_mod(def_id),
     def_const(def_id),