thread: Add Sync bound to ScopedKey (fixes #25894)

[Manishearth]

I have no clue what I'm doing. Someone should verify that this is
the necessary/sufficient fix to #25894 before merging 😛

[rust-highfive]

r? @aturon

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[Manishearth]

Running it on the original testcase gives us

test.rs:11:18: 11:24 error: the trait `core::marker::Sync` is not implemented for the type `alloc::rc::Rc<u8>` [E0277]
test.rs:11      let guard = scoped(|| {
                            ^~~~~~
note: in expansion of closure expansion
test.rs:9:22: 32:3 note: expansion site
test.rs:11:18: 11:24 note: `alloc::rc::Rc<u8>` cannot be shared between threads safely
test.rs:11      let guard = scoped(|| {
                            ^~~~~~
note: in expansion of closure expansion
test.rs:9:22: 32:3 note: expansion site
error: aborting due to previous error

which is what we wanted

[Manishearth]

This breaks the cell_allowed() test in the same file.

On that note, I'm skeptical as to why Cell should be allowed here in the first place. But I haven't figured out this API properly yet.

/home/manishearth/Mozilla/rust/src/libstd/thread/scoped_tls.rs:75:13: 75:44 error: the trait `core::marker::Sync` is not implemented for the type `core::cell::UnsafeCell<u32>` [E0277]
/home/manishearth/Mozilla/rust/src/libstd/thread/scoped_tls.rs:75             ::std::thread::ScopedKey::new();
                                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/manishearth/Mozilla/rust/src/libstd/thread/scoped_tls.rs:66:1: 87:2 note: in expansion of scoped_thread_local!
/home/manishearth/Mozilla/rust/src/libstd/thread/scoped_tls.rs:262:9: 262:53 note: expansion site
/home/manishearth/Mozilla/rust/src/libstd/thread/scoped_tls.rs:75:13: 75:44 note: `core::cell::UnsafeCell<u32>` cannot be shared between threads safely
/home/manishearth/Mozilla/rust/src/libstd/thread/scoped_tls.rs:75             ::std::thread::ScopedKey::new();
                                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/manishearth/Mozilla/rust/src/libstd/thread/scoped_tls.rs:66:1: 87:2 note: in expansion of scoped_thread_local!
/home/manishearth/Mozilla/rust/src/libstd/thread/scoped_tls.rs:262:9: 262:53 note: expansion site
rustc: x86_64-unknown-linux-gnu/stage1/test/rbmltest-x86_64-unknown-linux-gnu
rustc: x86_64-unknown-linux-gnu/stage1/test/alloctest-x86_64-unknown-linux-gnu
/home/manishearth/Mozilla/rust/src/liballoc/lib.rs:82:12: 82:18 warning: unused or unknown feature, #[warn(unused_features)] on by default
/home/manishearth/Mozilla/rust/src/liballoc/lib.rs:82 #![feature(unique)]

[alexcrichton]

I don't think this is what we want as one of the major uses of thread-local storage is non-Sync data. This API is unstable so there's no huge sense of urgency to fix it, so I'm going to close this in favor of discussion on #25894 instead.