Add roles declarations to allow safe coercions #175
                
     Merged
            
            
          
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This allows terms of type
ReactThis props state,ReactClass props,Ref a,RefHandler aandSyntheticEvent_ eventto be coerced to typeReactThis props' state',ReactClass props',Ref b,RefHandler bandSyntheticEvent_ event'whenCoercible props props',Coercible state state',Coercible a bandCoercible event event'hold, hence allowing the zero costcoerceto introduce and eliminate newtypes in components props and state, refs and their handlers and synthetic events for instance.