monaxyd.monads: Use nil? instead of (is x nil)

For this, we need hy master, as nil? is not available in 0.9.12.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
monaxyhd.monads/maybe-m: Simplify the drop-while

Since we are using nil? anyway, we can just pass the function to
drop-while, instead of wrapping it in another function.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>