Move statistics utils to common place as headers only

[AlexanderKalistratov]

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[github-actions]

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

[coveralls]

coverage: 72.236%. remained the same
when pulling 3d9ecdc on move_utils_to_headers_only
into 76f4360 on master.

[github-actions]

Array API standard conformance tests for dpnp=0.18.0dev0=py312he4f9c94_65 ran successfully.
Passed: 1197
Failed: 0
Skipped: 14

[AlexanderKalistratov]

@antonwolfy please review

[antonwolfy]

Thank you @AlexanderKalistratov for the improvement