Migrate assets pallet to new macros (#7984)

* Prep: move things around to suggested order

* Compiles, tests pass

* cleanup

* cleanup 2

* Fix dead doc-links

* Add back documentation for storage items

* Switch benchmarks to use `Event` rather than `RawEvent`.

* Update frame/assets/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* reviwe feedback

* Obey line length checks

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>