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/support/test/tests/pallet.rs

@@ -17,6 +17,7 @@
 
 use frame_support::{
 	dispatch::{Parameter, UnfilteredDispatchable},
+	pallet_prelude::ValueQuery,
 	storage::unhashed,
 	traits::{
 		ConstU32, GetCallName, GetStorageVersion, OnFinalize, OnGenesis, OnInitialize,
@@ -1631,3 +1632,17 @@ fn assert_type_all_pallets_without_system_reversed_is_correct() {
 		_a(t)
 	}
 }
+
+#[test]
+fn test_storage_alias() {
+	#[frame_support::storage_alias]
+	type Value<T: pallet::Config>
+	where
+		<T as frame_system::Config>::AccountId: From<SomeType1> + SomeAssociation1,
+	= StorageValue<pallet::Pallet<T>, u32, ValueQuery>;
+
+	TestExternalities::default().execute_with(|| {
+		pallet::Value::<Runtime>::put(10);
+		assert_eq!(10, Value::<Runtime>::get());
+	})
+}