Speed up CoefficientsQadic for long results

[frankluebeck]

This doesn't seem to be slower for small input, but is dramatically
faster for long return values (using a divide and conquer recursion).

Also added some calls of this operation in test file.

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Text for release notes

Improved CoefficientsQadic for long results.

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• usage of relevant labels

  1. either release notes: not needed or release notes: to be added
  2. at least one of the labels bug or enhancement or new feature
  3. for changes meant to be backported to stable-4.X add the backport-to-4.X label
  4. consider adding any of the labels build system, documentation, kernel, library, tests

• runnable tests

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• adequate pull request title

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• relevant documentation updates

• sensible comments in the code

[hulpke]

Nice!

[fingolfin]

Looks good to me, but it's not a bugfix, so I am not sure why it should be backported to 4.11?

... not that it matters at this point, as we might as well release 4.12.0 next anyway shrug we'll see in spring

[frankluebeck]

I've not added the "backport" label explicitly (just assumed that this was somehow added as default). Of course, I have an application for this and don't mind if the change goes into the next possible release.