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]

diff --cc src/compiletest/runtest.rs
Simple merge

diff --cc src/liballoc/arc.rs
Simple merge

diff --cc src/libcollections/dlist.rs
Simple merge

diff --cc src/libcore/atomic.rs
Simple merge

diff --cc src/librustc_driver/lib.rs
Simple merge

diff --cc src/librustc_trans/back/write.rs
Simple merge

diff --cc src/librustdoc/lib.rs
Simple merge

diff --cc src/librustdoc/test.rs
Simple merge

diff --cc src/libstd/io/comm_adapters.rs
Simple merge

diff --cc src/libstd/io/mod.rs
Simple merge

diff --cc src/libstd/io/process.rs
Simple merge

diff --cc src/libstd/macros.rs
Simple merge

diff --cc src/libstd/path/posix.rs
Simple merge

diff --cc src/libstd/path/windows.rs
Simple merge

diff --cc src/libstd/rand/os.rs
Simple merge

diff --cc src/libstd/sync/mpsc/mod.rs
Simple merge

diff --cc src/libstd/sync/mutex.rs
Simple merge

diff --cc src/libstd/sync/once.rs
Simple merge

diff --cc src/libstd/sync/rwlock.rs
Simple merge

diff --cc src/libstd/thread.rs
Simple merge

diff --cc src/libstd/thread_local/mod.rs
Simple merge

diff --cc src/libtest/lib.rs
Simple merge

diff --cc src/test/bench/shootout-fannkuch-redux.rs
Simple merge

diff --cc src/test/bench/shootout-k-nucleotide-pipes.rs
Simple merge

diff --cc src/test/bench/shootout-k-nucleotide.rs
Simple merge

diff --cc src/test/run-pass/logging-only-prints-once.rs
index 787d0e98e2bfcd79bd97337dc40c90b8f91a3a96,54f74239daf8603c81451cfade6a7ac748d4fc75..a72cfad2cb88ee27682d2506b93f848ddabd1174
--- 1/src/test/run-pass/logging-only-prints-once.rs
--- 2/src/test/run-pass/logging-only-prints-once.rs
+++ b/src/test/run-pass/logging-only-prints-once.rs
@@@ -27,9 -27,9 +27,9 @@@ impl fmt::Show for Foo 
  }
  
  pub fn main() {
-     Thread::spawn(move|| {
+     Thread::scoped(move|| {
          let mut f = Foo(Cell::new(0));
 -        println!("{}", f);
 +        println!("{:?}", f);
          let Foo(ref mut f) = f;
          assert!(f.get() == 1);
      }).join().ok().unwrap();

diff --cc src/test/run-pass/slice-panic-1.rs
Simple merge

diff --cc src/test/run-pass/slice-panic-2.rs
Simple merge