Missing types from `std` that we should probably support

[jonhoo]

• [ ] std::sync::Barrier
• [x] std::sync::atomic::AtomicI* integer types (the signed ones); done in #189
• [x] Atomic*::into_inner; done in #327
• [ ] Atomic*::get_mut (see also #154)
• [ ] std::sync::atomic::compiler_fence
• [x] std::sync::atomic::fence with SeqCst; done in #220
• [ ] UnsafeCell::raw_get (see also #276)
• [ ] std::sync::Weak and std::sync::Arc::downgrade (see also #156)
• [ ] Atomic*::as_ptr (see also #298)
• [ ] const block in thread_local (see also #346)
• [ ] LocalKey<Cell<T>>::* and LocalKey<RefCell<T>>::* (see also #347)

Currently unstable:

• [ ] std::sync::atomic::Atomic{I,U}128
• [ ] SyncUnsafeCell (see also #333)

[jonhoo]

Having the above would make https://github.com/crossbeam-rs/crossbeam/pull/487 a little nicer.

[taiki-e]

• [ ] std::sync::atomic::AtomicI* integer types (the signed ones)

Seems AtomicI* other than AtomicI128 was added in #189.

[anko]

I submitted https://github.com/tokio-rs/loom/issues/346 and https://github.com/tokio-rs/loom/issues/347 as separate issues. If they fit better here, feel free to add them to the list and close as duplicate.