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/staking/src/lib.rs

@@ -313,8 +313,6 @@ use sp_staking::{
 	SessionIndex,
 	offence::{OnOffenceHandler, OffenceDetails, Offence, ReportOffence, OffenceError},
 };
-#[cfg(feature = "std")]
-use sp_runtime::{Serialize, Deserialize};
 use frame_system::{
 	self as system, ensure_signed, ensure_root,
 	offchain::SendTransactionTypes,
@@ -380,7 +378,7 @@ pub struct EraRewardPoints<AccountId: Ord> {
 
 /// Indicates the initial status of the staker.
 #[derive(RuntimeDebug)]
-#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
 pub enum StakerStatus<AccountId> {
 	/// Chilling.
 	Idle,
@@ -793,7 +791,7 @@ pub trait Config: frame_system::Config + SendTransactionTypes<Call<Self>> {
 
 /// Mode of era-forcing.
 #[derive(Copy, Clone, PartialEq, Eq, Encode, Decode, RuntimeDebug)]
-#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
 pub enum Forcing {
 	/// Not forcing anything - just let whatever happen.
 	NotForcing,