Fix main thread requirements on protocols #509
                
     Merged
            
            
          
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In #359 I implemented marking classes as
MainThreadOnly, but a glaring omission there is that protocols were not yet marked in a similar way.This PR rectifies that by changing how the
mutabilitytraits work, allowing implementing them forProtocolObject<P>as well, which means we can now add "requirements" to protocols, such astrait MyProtocol: IsMainThreadOnly, ortrait MyProtocol: IsRetainable.We're still missing
IsRetainablebounds on pretty much everything inAppKit(and others) before we can make protocol overriding safe, but now at least the capability of adding those attributes is there.