Remove const qualifiers for class/struct members per C++ guidelines

[oleksandr-pavlyk]

Further remove const qualifier from struct/class members per C++ guidelines.

The test should be that each such class instance be device copyable. Perhaps static asserts to this effect can be added to the code.

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[github-actions]

Array API standard conformance tests for dpctl=0.19.0dev0=py310h93fe807_486 ran successfully.
Passed: 894
Failed: 2
Skipped: 118

[coveralls]

coverage: 88.18%. remained the same
when pulling 936fe68 on further-remove-const-members-of-class-and-structs
into 14a0cf4 on master.

[ndgrigorian]

LGTM! Honestly it's fewer than expected