Reverse argument order in R.tap. Solve #453

R.tap took its arguments value first, function second but it was
generally agreed that the function is more extensible if the arguments
were reversed.
i believe the only reason that type-check is there is legacy from when we were treating `tap` as variadic, way, way back when. now we expect the user to pass in the right stuff without us enforcing it. e.g. we don't check the `fn` passed to `map`, `foldl`, etc.