Make constexpr variables static or inline

[ndgrigorian]

This PR proposes changes throughout dpctl to make constexpr variables in namespaces in headers inline and other constexpr variables static

This results in some improvements to binary size and build time

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?
• If this PR is a work in progress, are you opening the PR as a draft?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2094/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_3 ran successfully.
Passed: 1115
Failed: 6
Skipped: 119

[coveralls]

coverage: 84.978% (-0.006%) from 84.984%
when pulling 6edcc21 on inline-static-constexpr-variables
into a26cac1 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_5 ran successfully.
Passed: 1114
Failed: 7
Skipped: 119