Updates for arity = 0 functions

Hi, I was searching for future library, and this one sounds nice.
While I was playing around it, I faced a BadArityError when using arity = 0 function, and tried to modify some. What do you think?

```
** (BadArityError) bad arity error: #Function<4.102239090 in FutureTest.test a future with no arguments/1> called with ()
```