soundness for capsule reference and name (#5474)

* Add safer methods to PyCapsule that check the capsule name

* soundness for `reference` and `name`

* remove `reference_checked`

* clippy

* review feedback

* SAFETY notes on unsafe blocks

* return capsule name as `CapsuleName` struct

* update newsfragment

* review feedback

* typo

* clippy

* fingers crossed...

* Update src/types/capsule.rs

Co-authored-by: Thomas Tanon <[email protected]>

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Co-authored-by: Jan Špaček <[email protected]>
Co-authored-by: Thomas Tanon <[email protected]>

Author: 3 people

newsfragments/5474.added.md

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+added checked methods into `PyCapsuleMethods`: `pointer_checked()`, `is_valid_checked()`

newsfragments/5474.changed.md

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+Deprecate unchecked `PyCapsuleMethods`: `pointer()`, `reference()`, `is_valid()`
+Reduce lifetime of return value in `PyCapsuleMethods::reference`
+`PyCapsuleMethods::name` now returns `CapsuleName` wrapper instead of `&CStr`.