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/construct_runtime/parse.rs

@@ -42,6 +42,7 @@ mod keyword {
 	syn::custom_keyword!(ValidateUnsigned);
 	syn::custom_keyword!(FreezeReason);
 	syn::custom_keyword!(HoldReason);
+	syn::custom_keyword!(Task);
 	syn::custom_keyword!(LockId);
 	syn::custom_keyword!(SlashReason);
 	syn::custom_keyword!(exclude_parts);
@@ -404,6 +405,7 @@ pub enum PalletPartKeyword {
 	ValidateUnsigned(keyword::ValidateUnsigned),
 	FreezeReason(keyword::FreezeReason),
 	HoldReason(keyword::HoldReason),
+	Task(keyword::Task),
 	LockId(keyword::LockId),
 	SlashReason(keyword::SlashReason),
 }
@@ -434,6 +436,8 @@ impl Parse for PalletPartKeyword {
 			Ok(Self::FreezeReason(input.parse()?))
 		} else if lookahead.peek(keyword::HoldReason) {
 			Ok(Self::HoldReason(input.parse()?))
+		} else if lookahead.peek(keyword::Task) {
+			Ok(Self::Task(input.parse()?))
 		} else if lookahead.peek(keyword::LockId) {
 			Ok(Self::LockId(input.parse()?))
 		} else if lookahead.peek(keyword::SlashReason) {
@@ -459,6 +463,7 @@ impl PalletPartKeyword {
 			Self::ValidateUnsigned(_) => "ValidateUnsigned",
 			Self::FreezeReason(_) => "FreezeReason",
 			Self::HoldReason(_) => "HoldReason",
+			Self::Task(_) => "Task",
 			Self::LockId(_) => "LockId",
 			Self::SlashReason(_) => "SlashReason",
 		}
@@ -471,7 +476,7 @@ impl PalletPartKeyword {
 
 	/// Returns the names of all pallet parts that allow to have a generic argument.
 	fn all_generic_arg() -> &'static [&'static str] {
-		&["Event", "Error", "Origin", "Config"]
+		&["Event", "Error", "Origin", "Config", "Task"]
 	}
 }
 
@@ -489,6 +494,7 @@ impl ToTokens for PalletPartKeyword {
 			Self::ValidateUnsigned(inner) => inner.to_tokens(tokens),
 			Self::FreezeReason(inner) => inner.to_tokens(tokens),
 			Self::HoldReason(inner) => inner.to_tokens(tokens),
+			Self::Task(inner) => inner.to_tokens(tokens),
 			Self::LockId(inner) => inner.to_tokens(tokens),
 			Self::SlashReason(inner) => inner.to_tokens(tokens),
 		}