contracts: Add `seal_rent_status` (#8780)

* Move public functions up in rent.rs

* Added RentStatus

* Fix test name for consistency

Co-authored-by: Michael Müller <[email protected]>

* Mark rent functions as unstable

* Add unstable interfaces to README

* Fix doc typos

Co-authored-by: Andrew Jones <[email protected]>

* Use DefaultNoBound

* Simplify calc_share(1)

* Don't output empty debug messages

* Make `seal_debug_message` unstable

Co-authored-by: Michael Müller <[email protected]>
Co-authored-by: Andrew Jones <[email protected]>