Fix risch_integrate(x**x, x) to work

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@cheatiiit, this would be a good review opportunity for you. Reviewing this
pull request would be a good exercise in getting familiar with the code in
DifferentialExtension, which you will need when you add the ability to
integrate tangents to the top-level risch_integrate(). Just go through the
code and try to understand what it does, and make sure that I didn't make any
mistakes.