Make some offset indexer classes constexpr

[oleksandr-pavlyk]

Making NoOpIndexer constructor and the call operator a constexpr allows compiler to completely inline its definition at earlier optimization stages.

Also replaced stray py::ssize_t with ssize_t.

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[github-actions]

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

[coveralls]

coverage: 91.145%. remained the same
when pulling 2c7609a on make-some-offset-classes-constexpr
into e2d9640 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.15.1dev3=py310h15de555_92 ran successfully.
Passed: 908
Failed: 1
Skipped: 86

[ndgrigorian]

LGTM, this is a smart change! I'd like to note here that the constexpr calls to the getters are enabled by the constexpr constructors.