Implement rename me -> self (#2444)

Implements change proposed in #1382

Replaces #1624

explorer/testdata/interface/impl_self_interface_parameter.carbon

@@ -10,7 +10,7 @@
 package ExplorerTest api;
 
 interface MyAddWith(T:! Type) {
-  fn Op[me: Self](b: T) -> Self;
+  fn Op[self: Self](b: T) -> Self;
 }
 
 class Point {
@@ -20,8 +20,8 @@ class Point {
 
 // Allowed: `Self` means `Point` after `as`
 impl Point as MyAddWith(Self) {
-  fn Op[me: Point](b: Point) -> Point {
-    return {.x = me.x + b.x, .y = me.y + b.y};
+  fn Op[self: Point](b: Point) -> Point {
+    return {.x = self.x + b.x, .y = self.y + b.y};
   }
 }