Implement rename me -> self (#2444)

Implements change proposed in #1382

Replaces #1624

explorer/testdata/generic_function/generic_method.carbon

@@ -12,13 +12,13 @@ package ExplorerTest api;
 class Cell(T:! Type) {
   fn Create(x: T) -> Cell(T) { return { .data = x }; }
 
-  fn Get[me: Self]() -> T {
-    return me.data;
+  fn Get[self: Self]() -> T {
+    return self.data;
   }
-  fn Put[addr me: Self*](x: T) {
-    (*me).data = x;
+  fn Put[addr self: Self*](x: T) {
+    (*self).data = x;
   }
-  fn CreateOther[me: Self, U:! Type](x: U) -> Cell(U) {
+  fn CreateOther[self: Self, U:! Type](x: U) -> Cell(U) {
     return {.data = x};
   }
   var data: T;

explorer/testdata/generic_function/implicit_conversion.carbon

@@ -10,10 +10,10 @@
 package ExplorerTest api;
 
 interface IntLike {
-  fn Convert[me: Self]() -> i32;
+  fn Convert[self: Self]() -> i32;
 }
 impl i32 as IntLike {
-  fn Convert[me: i32]() -> i32 { return me; }
+  fn Convert[self: i32]() -> i32 { return self; }
 }
 
 fn add[T:! IntLike](x: {.a: T, .b: ({.m: i32, .n: T}, i32)}) -> i32 {