Implement rename me -> self (#2444)

Implements change proposed in #1382

Replaces #1624

explorer/testdata/interface/fail_impl_missing_member.carbon

@@ -9,16 +9,16 @@
 package ExplorerTest api;
 
 interface Vector {
-  fn Add[me: Self](b: Self) -> Self;
-  fn Scale[me: Self](v: i32) -> Self;
+  fn Add[self: Self](b: Self) -> Self;
+  fn Scale[self: Self](v: i32) -> Self;
 }
 
 class Point {
   var x: i32;
   var y: i32;
   impl Point as Vector {
-    fn Add[me: Point](b: Point) -> Point {
-      return {.x = me.x + b.x, .y = me.y + b.y};
+    fn Add[self: Point](b: Point) -> Point {
+      return {.x = self.x + b.x, .y = self.y + b.y};
     }
   // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_missing_member.carbon:[[@LINE+1]]: implementation missing Scale
   }