Rewrite magic handler

[MarcoGorelli]

closes #594

[MarcoGorelli]

Did a bunch of integration tests, and this still passes. This results in less regular expressions, though it does mean giving up on processing Python code within magics, such as

%timeit foo = bar()

However, it does mean being able to handle

flake8_version = !flake8 --version

so IMO it's worth it

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EDIT

doesn't handle the latter case in the end. However, it is a lot simpler, much faster, and easier to maintain, so IMO this is still worth it

[MarcoGorelli]

Need to update the known limitations, improve some naming, and improve efficiency of the ast parsing (store a nested dict where the first layer just has the line number)

[girip11]

@MarcoGorelli You could merge this PR. I am unable to review it currently