Add ext_transfer call (#5169)

* contracts: Add ext_transfer call

This call allows contracts to send balance to any account
contract or not. Previously, the only way to do that was
though ext_call.

* Apply suggestions from code review

Co-Authored-By: Nikolay Volf <nikvolf@gmail.com>

* The define_env! macro does not allow for trailing comma

* Update frame/contracts/src/exec.rs

Co-Authored-By: Nikolay Volf <nikvolf@gmail.com>

* Bump spec version

* Do not use nested gas meter

* Use explicit 0 or 1 as return value

* Remove superflous intermediate binding

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

substrate/frame/contracts/src/exec.rs

@@ -120,6 +120,14 @@ pub trait Ext {
 		input_data: Vec<u8>,
 	) -> Result<(AccountIdOf<Self::T>, ExecReturnValue), ExecError>;
 
+	/// Transfer some amount of funds into the specified account.
+	fn transfer(
+		&mut self,
+		to: &AccountIdOf<Self::T>,
+		value: BalanceOf<Self::T>,
+		gas_meter: &mut GasMeter<Self::T>,
+	) -> Result<(), DispatchError>;
+
 	/// Call (possibly transferring some amount of funds) into the specified account.
 	fn call(
 		&mut self,
@@ -331,6 +339,23 @@ where
 		}
 	}
 
+	/// Transfer balance to `dest` without calling any contract code.
+	pub fn transfer(
+		&mut self,
+		dest: T::AccountId,
+		value: BalanceOf<T>,
+		gas_meter: &mut GasMeter<T>
+	) -> Result<(), DispatchError> {
+		transfer(
+			gas_meter,
+			TransferCause::Call,
+			&self.self_account.clone(),
+			&dest,
+			value,
+			self,
+		)
+	}
+
 	/// Make a call to the specified address, optionally transferring some funds.
 	pub fn call(
 		&mut self,
@@ -706,6 +731,15 @@ where
 		self.ctx.instantiate(endowment, gas_meter, code_hash, input_data)
 	}
 
+	fn transfer(
+		&mut self,
+		to: &T::AccountId,
+		value: BalanceOf<T>,
+		gas_meter: &mut GasMeter<T>,
+	) -> Result<(), DispatchError> {
+		self.ctx.transfer(to.clone(), value, gas_meter)
+	}
+
 	fn call(
 		&mut self,
 		to: &T::AccountId,