Function reader_t_fmap 

pub fn reader_t_fmap<E: 'static, M: FunctorSt + 'static, A: 'static, B: 'static>(
    fa: Box<dyn Fn(E) -> M::Of<A>>,
    f: impl Fn(A) -> B + 'static,
) -> Box<dyn Fn(E) -> M::Of<B>>

ReaderT fmap: apply a function to the result.