Migrate All Pallets to Attribute Macro

[songtianyi]

Here is the list of pallet and their status, if you want to migrate one you can put your name after it to avoid duplicated work.
some helpful guidelines to do the migration: https://crates.parity.io/frame_support/attr.pallet.html#upgrade-guidelines
some helpful guidelines to review a migration: https://crates.parity.io/frame_support/attr.pallet.html#checking-upgrade-guidelines

• darwinia-crab-backing @songtianyi feat: migrate crab backing to use pallet! #560
• darwinia-crab-issuing @songtianyi feat: migrate crab issuing to use pallet! #568
• darwinia-balances @AurevoirXavier Migrate balances to Attribute Macro #593
• darwinia-tron-backing @AurevoirXavier Migrate tron-backing to Attribute Macro #610
• darwinia-vesting @AurevoirXavier Substrate Updates #586
• darwinia-ethereum-backing @AurevoirXavier Migrate ethereum-backing to Attribute Macro #611
• darwinia-ethereum-issuing @AurevoirXavier
• darwinia-relayer-game
• darwinia-relay-authorities
• darwinia-staking @AurevoirXavier Migration darwinia-staking to Attribute Macro #924
• darwinia-header-mmr @AurevoirXavier Migrate header-mmr to Attribute Macro #613
• darwinia-evm @AsceticBear Upgrade EVM pallet to FRAMEv2 #648
• dvm-dynamic-fee @AurevoirXavier @AsceticBear Migrate dvm-dynamic-fee to Attribute Macro #644
• dvm-ethereum @AsceticBear Upgrade Ethereum pallet to FRAMEv2 #649

[AurevoirXavier]

And I propose this style:

// LICENSE

//! # Module Introduction

//(ignore this in real code) does't need to comment on `pub use` 
pub use pallet::*;
pub use weights::WeightInfo;

//(ignore this in real code) test module above others
#[cfg(test)]
mod mock;
#[cfg(test)]
mod tests;

#[frame_support::pallet]
pub mod pallet {
	//(ignore this in real code) pub mod types go first
	pub mod types {
		//(ignore this in real code) all import need a comment, easy for code review
		// --- darwinia ---
		use super::*;

		// Simple type
		pub type SimpleNum = u128;
		// Generic type
		pub type AccountId<T> = <T as frame_system::Config>::AccountId;
	}
	pub use types::*;

	//(ignore this in real code) imports go second
	//(ignore this in real code) all import need a comment, easy for code review
	// --- substrate ---
	use frame_support::pallet_prelude::*;
	use frame_system::pallet_prelude::*;
	//(ignore this in real code) all import need a comment, easy for code review
	// --- darwinia ---
	use crate::weights::WeightInfo;

	//(ignore this in real code) trait goes third
	#[pallet::config]
	pub trait Config: frame_system::Config {
		//(ignore this in real code) substrate framework types here
		// --- substrate ---
		type Event: From<Event<Self>> + IsType<<Self as frame_system::Config>::Event>;
		type WeightInfo: WeightInfo;
		//(ignore this in real code) custom types here
		// --- darwinia ---
		#[pallet::constant]
		type ModuleId: Get<ModuleId>;
	}

	//(ignore this in real code) event goes forth
	#[pallet::event]
	pub enum Event<T: Config> {
		//(ignore this in real code) need to place a `\` before the  bracket, cause these comment will generate the docs
		/// Dummy Event. \[who\]
		DummyEvent(AccountId<T>),
	}

	//(ignore this in real code) error goes fifth
	pub enum Error<T> {}

	//(ignore this in real code) storages go sixth
	//(ignore this in real code) keep visibility simple as `pub` or empty, if necessary then `pub (sup)` or `pub (crate)`
	#[pallet::storage]
	#[pallet::getter(fn total_mapped_ring)]
	pub type DummyStorage<T: Config> = StorageValue<_, SimpleNum>;

	//(ignore this in real code) genesis config goes seventh, if this is a complex struct `impl Default` manually
	#[cfg_attr(feature = "std", derive(Default))]
	#[pallet::genesis_config]
	pub struct GenesisConfig {
		pub total_mapped_ring: MappedRing,
	}
	//(ignore this in real code) and I recommend not to leave a empty line between struct and its own impl
	#[pallet::genesis_build]
	impl<T: Config> GenesisBuild<T> for GenesisConfig {
		fn build(&self) {}
	}

	//(ignore this in real code) dispatch calls go last
	#[pallet::pallet]
	pub struct Pallet<T>(_);
	//(ignore this in real code) and I recommend not to leave a empty line between struct and its own impl
	#[pallet::hooks]
	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
	//(ignore this in real code) and I recommend not to leave a empty line between struct and its own impl
	#[pallet::call]
	impl<T: Config> Pallet<T> {}
}

[AurevoirXavier]

Some pallets will be removed later.