Suggestion: DeepReadonly<T> type

TypeScript Version: 2.1.1 / nightly (2.2.0-dev.201xxxxx)

Code

It would be nice to have a shard, standard library type that allows to express deep readonly-ness (not really const, since methods are out of scope, but still...):

interface Y { a: number; }
interface X { y: Y; }
let x: Readonly<X> = {y: {a: 1}};
x.y.a = 2;  // Succeeds, which is expected, but it'd be nice to have a common way to express deep readonly

type DeepReadonly<T> = {
  readonly [P in keyof T]: DeepReadonly<T[P]>;
}
let deepX: DeepReadonly<X> = {y: {a: 1}};
deepX.y.a = 2; // Fails as expected!

Start with the issue's DeepReadonly example and review how TypeScript 2.1 mapped and recursive types behave. The requested outcome is a commonly available deep-readonly type whose nested properties reject assignment, but the issue does not name a file, test, or settled implementation path; confirm the intended standard-library design and add coverage for the shown nested-object case.