Remove serde requirement from FRAME macros (#8628)

* Remove `serde` requirement from FRAME macros

Currently there is some implicit requirement on `serde` being present in
the `Cargo.toml` of a pallet when `GenesisConfig` is used. This pr
removes this requirement by using the serde attribute `serde(crate = "..")`.

* build a unique reexport of serde in impl_opaque_keys, by abusing paste doc concatenation

* Optimize

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

substrate/frame/support/procedural/src/storage/genesis_config/mod.rs

@@ -65,6 +65,7 @@ fn decl_genesis_config_and_impl_default(
 	let genesis_struct = &genesis_config.genesis_struct;
 	let genesis_impl = &genesis_config.genesis_impl;
 	let genesis_where_clause = &genesis_config.genesis_where_clause;
+	let serde_crate = format!("{}::serde", scrate);
 
 	quote!(
 		/// Genesis config for the module, allow to build genesis storage.
@@ -72,6 +73,7 @@ fn decl_genesis_config_and_impl_default(
 		#[cfg(feature = "std")]
 		#[serde(rename_all = "camelCase")]
 		#[serde(deny_unknown_fields)]
+		#[serde(crate = #serde_crate)]
 		#serde_bug_bound
 		pub struct GenesisConfig#genesis_struct_decl #genesis_where_clause {
 			#( #config_fields )*