pallet-contracts: Refactor and comment `rent` module. (#4733)

* Refactor and comment `rent` module.

* impl_version bump

* Add doc for Exempt

* Simplify code.

* Update bin/node/runtime/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* Update frame/contracts/src/exec.rs

Co-Authored-By: Hero Bird <robin.freyler@gmail.com>

Co-authored-by: thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Hero Bird <robbepop@web.de>