Auto merge of #21165 - alexcrichton:second-pass-type-id, r=aturon

This commit aims to stabilize the `TypeId` abstraction by moving it out of the
`intrinsics` module into the `any` module of the standard library. Specifically,

* `TypeId` is now defined at `std::any::TypeId`
* `TypeId::hash` has been removed in favor of an implementation of `Hash`.

This commit also performs a final pass over the `any` module, confirming the
following:

* `Any::get_type_id` remains unstable as *usage* of the `Any` trait will likely
  never require this, and the `Any` trait does not need to be implemented for
  any other types. As a result, this implementation detail can remain unstable
  until associated statics are implemented.
* `Any::downcast_ref` is now stable
* `Any::downcast_mut` is now stable
* `BoxAny` remains unstable. While a direct impl on `Box<Any>` is allowed today
  it does not allow downcasting of trait objects like `Box<Any + Send>` (those
  returned from `Thread::join`). This is covered by #18737.
* `BoxAny::downcast` is now stable.

src/test/compile-fail/extern-with-type-bounds.rs

@@ -10,11 +10,9 @@
 
 #![feature(intrinsics)]
 
-use std::intrinsics::TypeId;
-
 extern "rust-intrinsic" {
     // Real example from libcore
-    fn type_id<T: ?Sized + 'static>() -> TypeId;
+    fn type_id<T: ?Sized + 'static>() -> u64;
 
     // Silent bounds made explicit to make sure they are actually
     // resolved.

src/test/compile-fail/issue-18389.rs

@@ -11,7 +11,7 @@
 #![feature(unboxed_closures)]
 
 use std::any::Any;
-use std::intrinsics::TypeId;
+use std::any::TypeId;
 
 pub trait Pt {}
 pub trait Rt {}