Realias DiffResults whenever possible

[gdalle]

Fix #251 partially.

• [x] Do result = operator!(result, f, x) instead of operator!(result, f, x) whenever possible (see https://github.com/JuliaDiff/DiffResults.jl/issues/27)
• [ ] There are still some parts involving closures which I don't know how to handle.
• [x] Adjust the tests accordingly
• [ ] Add new tests that would fail on the current release.

@devmotion do you know the package well enough to help me figure out what I missed?

[devmotion]

I'm off until next week and probably won't be able to add a meaningful comment until then.

[gdalle]

No worries, I'll wait for your opinion!