* generate_storage_alias: Rewrite as proc macro attribute

This rewrites the `generate_storage_alias!` declarative macro as proc-macro attribute. While doing
this the name is changed to `storage_alias`. The prefix can now also be the name of a pallet. This
makes storage aliases work in migrations for all kind of chains and not just for the ones that use
predefined prefixes.

* Fix compilation and FMT

* Moare fixes

* 🤦

* ......

* Rework the syntax and support instancing

* FMT

* Prefix variants with `Storage`

* Make it compile

* Fix where clause on rust stable