Correct handling of walrus operator in function args

[thatch]

Summary

Previous behavior treated it as identical to equal, making a kwarg; it should
instead be a positional arg. Includes several tests to make sure that
whitespace handling is correct.

Fixes #416

Test Plan

Includes new tests that were failing before. I had some trouble ensuring the whitespace ended up in the right spot during development, so this includes a few probably unnecessary tests out of a general distrust that I did it right.

[amyreese]

Looks good to me, and I verified locally that it resolves the issue from usort.

Before fix:

(.venv) jreese@garbodor ~/workspace/usort main± » cat foo.py
foo.func(value := "some string literal")

(.venv) jreese@garbodor ~/workspace/usort main± » usort diff foo.py
--- a/foo.py
+++ b/foo.py
@@ -1,2 +1,2 @@
-foo.func(value := "some string literal")
+foo.func(value = "some string literal")

With fix:

(.venv) jreese@garbodor ~/workspace/usort main± » pip install -U git+https://github.com/thatch/libcst@issue416
Collecting git+https://github.com/thatch/libcst@issue416
  Cloning https://github.com/thatch/libcst (to revision issue416) to /private/var/folders/_b/6gpzshq548d4d_4bk1m5sq280000gn/T/pip-req-build-n3b3irzn
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Requirement already satisfied, skipping upgrade: typing-inspect>=0.4.0 in ./.venv/lib/python3.8/site-packages (from libcst==0.3.13) (0.6.0)
Requirement already satisfied, skipping upgrade: typing-extensions>=3.7.4.2 in ./.venv/lib/python3.8/site-packages (from libcst==0.3.13) (3.7.4.3)
Requirement already satisfied, skipping upgrade: pyyaml>=5.2 in ./.venv/lib/python3.8/site-packages (from libcst==0.3.13) (5.3.1)
Requirement already satisfied, skipping upgrade: mypy-extensions>=0.3.0 in ./.venv/lib/python3.8/site-packages (from typing-inspect>=0.4.0->libcst==0.3.13) (0.4.3)
Building wheels for collected packages: libcst
  Building wheel for libcst (PEP 517) ... done
  Created wheel for libcst: filename=libcst-0.3.13-py3-none-any.whl size=502635 sha256=9dd3fcfe16688992f2ff6bb0ddd8235f15fb595134768bbdb9aeb5660fcee259
  Stored in directory: /private/var/folders/_b/6gpzshq548d4d_4bk1m5sq280000gn/T/pip-ephem-wheel-cache-i73lgx32/wheels/cc/af/27/cece6ddd4a6a62f85a0ea3964b00c4fd3d176216786b578d03
Successfully built libcst
Installing collected packages: libcst
  Attempting uninstall: libcst
    Found existing installation: libcst 0.3.13
    Uninstalling libcst-0.3.13:
      Successfully uninstalled libcst-0.3.13
Successfully installed libcst-0.3.13
WARNING: You are using pip version 20.2.1; however, version 20.2.4 is available.
You should consider upgrading via the '/Users/jreese/workspace/usort/.venv/bin/python -m pip install --upgrade pip' command.

(.venv) jreese@garbodor ~/workspace/usort main± » usort diff foo.py

[jimmylai]

Other than the comment, LGTM.

[jimmylai]

We don't use assert since it's noop in optimized mode. Let's just use if condition and raise when it returns unexpected result.

Suggested change

	assert isinstance(val, WithLeadingWhitespace)
	if isinstance(val, WithLeadingWhitespace):
	return Arg(value=val.value)
	else:
	raise Exception("...")