Anonymous named tuples

The syntax
```python
x = (a=1, b=2)
```
should compile to something like
```python
x = namedtuple("_", ("a", "b"))(1, 2)
```
and the annotated version
```python
x = (a=1, b: int = 2)
```
should compile to something like
```python
x = NamedTuple("_", [("a", "Any"), ("b", "int")])(1, 2)
```