Missing types from std that we should probably support

[jonhoo]

• std::sync::Barrier
• std::sync::atomic::AtomicI* integer types (the signed ones); done in Prepare 0.4 and add I variant atomic ints #189
• Atomic*::into_inner; done in Add Atomic*::into_inner #327
• Atomic*::get_mut (see also No get_mut method on AtomicPtr #154)
• std::sync::atomic::compiler_fence
• std::sync::atomic::fence with SeqCst; done in Implement Release, AcqRel, and SeqCst fences #220
• UnsafeCell::raw_get (see also Missing UnsafeCell::raw_get #276)
• std::sync::Weak and std::sync::Arc::downgrade (see also Missing std::sync::Weak #156)
• Atomic*::as_ptr (see also What to do when loom::AtomicUsize do not implement as_mut_ptr() #298)
• const block in thread_local (see also loom::thread_local does not support const-initialization #346)
• LocalKey<Cell<T>>::* and LocalKey<RefCell<T>>::* (see also loom::LocalKey does not support specialized implementations for LocalKey<Cell<T>> or LocalKey<RefCell<T>> #347)

Currently unstable:

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

[jonhoo]

Having the above would make crossbeam-rs/crossbeam#487 a little nicer.

[anko]

I submitted #346 and #347 as separate issues. If they fit better here, feel free to add them to the list and close as duplicate.