add static checking for observer functions
add static checking for observer functions

alexeymarkov/RxCpp,alexeymarkov/RxCpp,Reactive-Extensions/RxCpp,ReactiveX/RxCpp,ReactiveX/RxCpp,Reactive-Extensions/RxCpp,alexeymarkov/RxCpp,ReactiveX/RxCpp,Reactive-Extensions/RxCpp