Update `seal_call` to `__unstable__` version (#960)

* Update `seal_call` to `__unstable__` version

* Add `Debug` derive

* Make `cargo-spellcheck` happy

* Make `into_u32` take `self`

* Update release notes

* Fix crate docs link

* Use type-system encoded version of empty `ExecutionInput`

* Add `const` for getters and setters

* Shorten if-branches logic

* Update release notes

* Remove dead code

* Improve release notes