Implement rename me -> self (#2444)

Implements change proposed in #1382

Replaces #1624

explorer/testdata/interface/constrained_parameter.carbon

@@ -10,16 +10,16 @@
 package ExplorerTest api;
 
 interface AddMul {
-  fn Add[me: Self](o: Self) -> Self;
-  fn Mul[me: Self](o: Self) -> Self;
+  fn Add[self: Self](o: Self) -> Self;
+  fn Mul[self: Self](o: Self) -> Self;
 }
 
 external impl i32 as AddMul {
-  fn Add[me: i32](o: i32) -> i32 {
-    return me + o;
+  fn Add[self: i32](o: i32) -> i32 {
+    return self + o;
   }
-  fn Mul[me: i32](o: i32) -> i32 {
-    return me * o;
+  fn Mul[self: i32](o: i32) -> i32 {
+    return self * o;
   }
 }
 
@@ -29,9 +29,9 @@ class Holder(T:! AddMul) {
 
 interface Vector(Scalar:! AddMul) {
   fn Zero() -> Self;
-  fn Add[me: Self](b: Self) -> Self;
-  fn Scale[me: Self](v: Scalar) -> Self;
-  fn Hold[me: Self](v: Scalar) -> Holder(Scalar);
+  fn Add[self: Self](b: Self) -> Self;
+  fn Scale[self: Self](v: Scalar) -> Self;
+  fn Hold[self: Self](v: Scalar) -> Holder(Scalar);
 }
 
 class Point {
@@ -41,13 +41,13 @@ 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};
     }
-    fn Hold[me: Point](v: i32) -> Holder(i32) {
+    fn Hold[self: Point](v: i32) -> Holder(i32) {
       return {.v = v};
     }
   }