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/support/procedural/src/pallet/parse/composite.rs

@@ -26,11 +26,14 @@ pub mod keyword {
 	syn::custom_keyword!(HoldReason);
 	syn::custom_keyword!(LockId);
 	syn::custom_keyword!(SlashReason);
+	syn::custom_keyword!(Task);
+
 	pub enum CompositeKeyword {
 		FreezeReason(FreezeReason),
 		HoldReason(HoldReason),
 		LockId(LockId),
 		SlashReason(SlashReason),
+		Task(Task),
 	}
 
 	impl ToTokens for CompositeKeyword {
@@ -41,6 +44,7 @@ pub mod keyword {
 				HoldReason(inner) => inner.to_tokens(tokens),
 				LockId(inner) => inner.to_tokens(tokens),
 				SlashReason(inner) => inner.to_tokens(tokens),
+				Task(inner) => inner.to_tokens(tokens),
 			}
 		}
 	}
@@ -56,6 +60,8 @@ pub mod keyword {
 				Ok(Self::LockId(input.parse()?))
 			} else if lookahead.peek(SlashReason) {
 				Ok(Self::SlashReason(input.parse()?))
+			} else if lookahead.peek(Task) {
+				Ok(Self::Task(input.parse()?))
 			} else {
 				Err(lookahead.error())
 			}
@@ -71,6 +77,7 @@ pub mod keyword {
 				match self {
 					FreezeReason(_) => "FreezeReason",
 					HoldReason(_) => "HoldReason",
+					Task(_) => "Task",
 					LockId(_) => "LockId",
 					SlashReason(_) => "SlashReason",
 				}
@@ -80,7 +87,7 @@ pub mod keyword {
 }
 
 pub struct CompositeDef {
-	/// The index of the HoldReason item in the pallet module.
+	/// The index of the CompositeDef item in the pallet module.
 	pub index: usize,
 	/// The composite keyword used (contains span).
 	pub composite_keyword: keyword::CompositeKeyword,