DIP 1024--Shared Atomics--Community Review Round 1

[Kagamin]

Access to shared memory should be disallowed only for safe code, 
but not for system code. For safe code it's justified, because 
safety is unavoidable there, for system code there's a concern of 
ease of use of shared data, which is affected by littering and is 
not addressed in this DIP. Stripping shared qualifier off for 
assignment isn't necessarily correct if, say, the data type 
switches between atomic and nonatomic reference counting based on 
its type.