Support walrus operator assignments

...such as this:

```
if (some_key := unpack_keys(dict(some_key=42))):
print(some_key)
```

currently this raises

```
Traceback (most recent call last):
File "", line 1, in
File "/usr/local/lib/python3.11/site-packages/sorcery/core.py", line 185, in __call__
assert executing.node, "Failed to find call node"
AssertionError: Failed to find call node
```

while it would be magical if `sorcery` could handle this walrus assignment expression, too, wouldn't it?