Function twice

twice(func: Function): Function
Returns a new function which when applied to an argument, has the same effect as applying the specified function to the same argument 2 times.
Parameters
- func: Function
  the function to be repeated
Returns Function
the new function that has the same effect as (x => func(func(x)))
Example
```
const plusTwo = twice(x => x + 1);
plusTwo(2); // Returns 4
```