Move Functors to dedicated kernels folder

[antonwolfy]

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• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2816/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.20.0dev4=py313h509198e_4 ran successfully.
Passed: 1354
Failed: 3
Skipped: 16

[coveralls]

coverage: 81.225% (+0.06%) from 81.17%
when pulling 5935566 on move-functors-to-kernels-dir
into 4b64466 on master.