Add currrying decorator.

Add ``curried`` decorator, the function is called as soon as the arguments are all given:

```python
>>> from fn.func import curried
>>> @curried
... def sum5(a, b, c, d, e):
...     return a + b + c + d + e
...
>>> sum5(1)(2)(3)(4)(5)
15
>>> sum5(1, 2, 3)(4, 5)
15
```