rollup merge of #20615: aturon/stab-2-thread

This commit takes a first pass at stabilizing `std::thread`:

* It removes the `detach` method in favor of two constructors -- `spawn`
  for detached threads, `scoped` for "scoped" (i.e., must-join)
  threads. This addresses some of the surprise/frustrating debug
  sessions with the previous API, in which `spawn` produced a guard that
  on destruction joined the thread (unless `detach` was called).

  The reason to have the division in part is that `Send` will soon not
  imply `'static`, which means that `scoped` thread creation can take a
  closure over *shared stack data* of the parent thread. On the other
  hand, this means that the parent must not pop the relevant stack
  frames while the child thread is running. The `JoinGuard` is used to
  prevent this from happening by joining on drop (if you have not
  already explicitly `join`ed.) The APIs around `scoped` are
  future-proofed for the `Send` changes by taking an additional lifetime
  parameter. With the current definition of `Send`, this is forced to be
  `'static`, but when `Send` changes these APIs will gain their full
  flexibility immediately.

  Threads that are `spawn`ed, on the other hand, are detached from the
  start and do not yield an RAII guard.

  The hope is that, by making `scoped` an explicit opt-in with a very
  suggestive name, it will be drastically less likely to be caught by a
  surprising deadlock due to an implicit join at the end of a scope.

* The module itself is marked stable.

* Existing methods other than `spawn` and `scoped` are marked stable.

The migration path is:

```rust
Thread::spawn(f).detached()
```

becomes

```rust
Thread::spawn(f)
```

while

```rust
let res = Thread::spawn(f);
res.join()
```

becomes

```rust
let res = Thread::scoped(f);
res.join()
```

[breaking-change]

src/liballoc/arc.rs

@@ -42,7 +42,7 @@
 //!
 //!     Thread::spawn(move || {
 //!         println!("{}", five);
-//!     }).detach();
+//!     });
 //! }
 //! ```
 //!
@@ -63,7 +63,7 @@
 //!         *number += 1;
 //!
 //!         println!("{}", *number); // prints 6
-//!     }).detach();
+//!     });
 //! }
 //! ```
 
@@ -106,7 +106,7 @@ use heap::deallocate;
 ///             let local_numbers = child_numbers.as_slice();
 ///
 ///             // Work with the local numbers
-///         }).detach();
+///         });
 ///     }
 /// }
 /// ```