Tasks: general system for recognizing and executing service work (#1343)

`polkadot-sdk` version of original tasks PR located here:
https://github.com/paritytech/substrate/pull/14329

Fixes #206

## Status
- [x] Generic `Task` trait
- [x] `RuntimeTask` aggregated enum, compatible with
`construct_runtime!`
- [x] Casting between `Task` and `RuntimeTask` without needing `dyn` or
`Box`
- [x] Tasks Example pallet
- [x] Runtime tests for Tasks example pallet
- [x] Parsing for task-related macros
- [x] Retrofit parsing to make macros optional
- [x] Expansion for task-related macros
- [x] Adds support for args in tasks
- [x] Retrofit tasks example pallet to use macros instead of manual
syntax
- [x] Weights
- [x] Cleanup
- [x] UI tests
- [x] Docs

## Target Syntax
Adapted from
https://github.com/paritytech/polkadot-sdk/issues/206#issue-1865172283

```rust
// NOTE: this enum is optional and is auto-generated by the other macros if not present
#[pallet::task]
pub enum Task<T: Config> {
    AddNumberIntoTotal {
        i: u32,
    }
}

/// Some running total.
#[pallet::storage]
pub(super) type Total<T: Config<I>, I: 'static = ()> =
StorageValue<_, (u32, u32), ValueQuery>;

/// Numbers to be added into the total.
#[pallet::storage]
pub(super) type Numbers<T: Config<I>, I: 'static = ()> =
StorageMap<_, Twox64Concat, u32, u32, OptionQuery>;

#[pallet::tasks_experimental]
impl<T: Config<I>, I: 'static> Pallet<T, I> {
	/// Add a pair of numbers into the totals and remove them.
	#[pallet::task_list(Numbers::<T, I>::iter_keys())]
	#[pallet::task_condition(|i| Numbers::<T, I>::contains_key(i))]
	#[pallet::task_index(0)]
	pub fn add_number_into_total(i: u32) -> DispatchResult {
		let v = Numbers::<T, I>::take(i).ok_or(Error::<T, I>::NotFound)?;
		Total::<T, I>::mutate(|(total_keys, total_values)| {
			*total_keys += i;
			*total_values += v;
		});
		Ok(())
	}
}
```

---------

Co-authored-by: Nikhil Gupta <17176722+gupnik@users.noreply.github.com>
Co-authored-by: kianenigma <kian@parity.io>
Co-authored-by: Nikhil Gupta <>
Co-authored-by: Gavin Wood <gavin@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: gupnik <nikhilgupta.iitk@gmail.com>

substrate/frame/examples/tasks/src/lib.rs

@@ -0,0 +1,78 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! This pallet demonstrates the use of the `pallet::task` api for service work.
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use frame_support::dispatch::DispatchResult;
+// Re-export pallet items so that they can be accessed from the crate namespace.
+pub use pallet::*;
+
+pub mod mock;
+pub mod tests;
+
+#[cfg(feature = "runtime-benchmarks")]
+mod benchmarking;
+
+pub mod weights;
+pub use weights::*;
+
+#[frame_support::pallet(dev_mode)]
+pub mod pallet {
+	use super::*;
+	use frame_support::pallet_prelude::*;
+
+	#[pallet::error]
+	pub enum Error<T> {
+		/// The referenced task was not found.
+		NotFound,
+	}
+
+	#[pallet::tasks_experimental]
+	impl<T: Config> Pallet<T> {
+		/// Add a pair of numbers into the totals and remove them.
+		#[pallet::task_list(Numbers::<T>::iter_keys())]
+		#[pallet::task_condition(|i| Numbers::<T>::contains_key(i))]
+		#[pallet::task_weight(T::WeightInfo::add_number_into_total())]
+		#[pallet::task_index(0)]
+		pub fn add_number_into_total(i: u32) -> DispatchResult {
+			let v = Numbers::<T>::take(i).ok_or(Error::<T>::NotFound)?;
+			Total::<T>::mutate(|(total_keys, total_values)| {
+				*total_keys += i;
+				*total_values += v;
+			});
+			Ok(())
+		}
+	}
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config {
+		type RuntimeTask: frame_support::traits::Task;
+		type WeightInfo: WeightInfo;
+	}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(_);
+
+	/// Some running total.
+	#[pallet::storage]
+	pub type Total<T: Config> = StorageValue<_, (u32, u32), ValueQuery>;
+
+	/// Numbers to be added into the total.
+	#[pallet::storage]
+	pub type Numbers<T: Config> = StorageMap<_, Twox64Concat, u32, u32, OptionQuery>;
+}