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/procedural/src/lib.rs

@@ -31,6 +31,7 @@ mod pallet;
 mod pallet_error;
 mod partial_eq_no_bound;
 mod storage;
+mod storage_alias;
 mod transactional;
 mod tt_macro;
 
@@ -575,3 +576,10 @@ pub fn derive_pallet_error(input: TokenStream) -> TokenStream {
 pub fn __create_tt_macro(input: TokenStream) -> TokenStream {
 	tt_macro::create_tt_return_macro(input)
 }
+
+#[proc_macro_attribute]
+pub fn storage_alias(_: TokenStream, input: TokenStream) -> TokenStream {
+	storage_alias::storage_alias(input.into())
+		.unwrap_or_else(|r| r.into_compile_error())
+		.into()
+}