refactor: exclude encoding of reading state results in chain extension

[Daanvdplas]

Because we funnelled the read state functions through read_state on the contract size the result had to be raw bytes, therefore encoded on the runtime and decoded in the contract. The read state functions are now called specifically and therefore the result can be its type (e.g. Balance). By refactoring it prevents the pop api having to decode the read state results which can decrease the contract size.

With the current contract it decreases it from 7.1K to 6.8K (don't compare with the psp22 contract as the logic implemented is not equal).

In stead of:

fn read_fungibles_state<T, E>(
	key: Keys<T>,
	env: &mut Environment<E, BufInBufOutState>,
) -> Result<Vec<u8>, DispatchError>
where
	T: pallet_contracts::Config
		+ pallet_assets::Config<TrustBackedAssetsInstance, AssetId = AssetId>
		+ fungibles::Config,
	E: Ext<T = T>,
	T: frame_system::Config<AccountId = sp_runtime::AccountId32>,
{
	env.charge_weight(T::DbWeight::get().reads(1_u64))?;
	match key {
		TotalSupply(id) => Ok(fungibles::Pallet::<T>::total_supply(id).encode()),
	}
	...
}

We do:

fn read_fungibles_state<T, E>(
	key: Keys<T>,
	env: &mut Environment<E, BufInBufOutState>,
) -> Result<(), DispatchError>
where
	T: pallet_contracts::Config
		+ pallet_assets::Config<TrustBackedAssetsInstance, AssetId = AssetId>
		+ fungibles::Config,
	E: Ext<T = T>,
	T: frame_system::Config<AccountId = sp_runtime::AccountId32>,
{
	env.charge_weight(T::DbWeight::get().reads(1_u64))?;
	match key {
		TotalSupply(id) => env.write(&fungibles::Pallet::<T>::total_supply(id), false, None),
	}
	...
	Ok(())
}

[Daanvdplas]

Closed by #122