`nonlocal` raise `SyntaxError` when it is declared multiple times.

**CPython 3.7.4**
```python
def test_nonlocal():
    x = 0
    y = 0
    def f():
        nonlocal x
        nonlocal x, y

def test_nonlocal2():
    x = 0
    def f():
        nonlocal x
        nonlocal x
```
are OK.

**RustPython** gives `SyntaxError: name 'x' is used prior to nonlocal declaration at line 0 column 0` for both of the examples.


