Support walrus operator assignments

[larsblumberg]

...such as this:

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

currently this raises

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  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?