generate_storage_alias: Rewrite as proc macro attribute (#11387)

* 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

substrate/frame/offences/src/migration.rs

@@ -16,9 +16,7 @@
 // limitations under the License.
 
 use super::{Config, OffenceDetails, Perbill, SessionIndex};
-use frame_support::{
-	generate_storage_alias, pallet_prelude::ValueQuery, traits::Get, weights::Weight,
-};
+use frame_support::{pallet_prelude::ValueQuery, storage_alias, traits::Get, weights::Weight};
 use sp_staking::offence::{DisableStrategy, OnOffenceHandler};
 use sp_std::vec::Vec;
 
@@ -31,10 +29,9 @@ type DeferredOffenceOf<T> = (
 
 // Deferred reports that have been rejected by the offence handler and need to be submitted
 // at a later time.
-generate_storage_alias!(
-	Offences,
-	DeferredOffences<T: Config> => Value<Vec<DeferredOffenceOf<T>>, ValueQuery>
-);
+#[storage_alias]
+type DeferredOffences<T: Config> =
+	StorageValue<crate::Pallet<T>, Vec<DeferredOffenceOf<T>>, ValueQuery>;
 
 pub fn remove_deferred_storage<T: Config>() -> Weight {
 	let mut weight = T::DbWeight::get().reads_writes(1, 1);