Implement rename me -> self (#2444)

Implements change proposed in #1382

Replaces #1624

explorer/testdata/interface/class_function.carbon

@@ -11,8 +11,8 @@ package ExplorerTest api;
 
 interface Vector {
   fn Zero() -> Self;
-  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 {
@@ -22,11 +22,11 @@ class Point {
     fn Zero() -> Point {
       return {.x = 0, .y = 0};
     }
-    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};
     }
-    fn Scale[me: Point](v: i32) -> Point {
-      return {.x = me.x * v, .y = me.y * v};
+    fn Scale[self: Point](v: i32) -> Point {
+      return {.x = self.x * v, .y = self.y * v};
     }
   }
 }