Implement when function

[bannzai]

I implement "when" method.
It corresponds to multiple value type.

Usage

• success pattern

lettask1=Task<Float,String,NSError?>{ fulfill, reject infulfill("Success")}lettask2=Task<Float,Int,NSError?>{ fulfill, reject infulfill(1)}lettask3=Task<Float,Double,NSError?>{ fulfill, reject infulfill(1.1)}Task<Float,(String,Int,Double),NSError>.when((task1, task2, task3)).success{(string, int, double)->Voidinprint(string) // -> Success
print(int) // -> 1
print(double) // -> 1.1
}.failure{(error, isCancelled)->VoidinfatalError()}

*failure pattern

lettask1=Task<Float,String,String>{ fulfill, reject infulfill("Success")}lettask2=Task<Float,Int,String>{ fulfill, reject infulfill(1)}lettask3=Task<Float,Double,String>{ fulfill, reject inreject("Rejected")}Task<Float,(String,Int,Double),String>.when((task1, task2, task3)).success{(string, int, double)->VoidinfatalError()}.failure{(error, isCancelled)->Voidinprint(error) // -> Optional("Rejected")
}

[inamiy]

Hi, thank you for pull request :)

Currently, there is already Task.all() class method to achieve similar functionality.
For type-strict version, you can find scratch code in

SwiftTask/SwiftTask/SwiftTask.swift

Lines 669 to 699 in 4bdab82

	publicfunc zip<Progress2, Value2, Error2>(task2:Task<Progress2,Value2,Error2>)->Task<(Progress?,Progress2?),(Value,Value2),(Error?,Error2?)>
	{
	returnTask<(Progress?,Progress2?),(Value,Value2),(Error?,Error2?)>{ machine, progress, fulfill, _reject, configure in
	lett1=self.convert(progress: _toAny, value: _toAny, error: _toAny)
	lett2= task2.convert(progress: _toAny, value: _toAny, error: _toAny)
	letzipTask=Task<Any,Any,Any>.all([t1, t2])
	configure.pause ={ zipTask.pause()}
	configure.resume ={ zipTask.resume()}
	configure.cancel ={ zipTask.cancel()}
	t1.progress{ _, p in
	if t2.value ==nil && t2.errorInfo ==nil{
	progress((p as?Progress, t2.progress as?Progress2))
	}
	}
	t2.progress{ _, p in
	if t1.value ==nil && t1.errorInfo ==nil{
	progress((t1.progress as?Progress, p as?Progress2))
	}
	}
	zipTask.success{(valueTuple:[Any])->Voidin
	fulfill((valueTuple[0]as!Value,valueTuple[1]as!Value2))
	}.failure{ _, isCancelled ->Voidin
	_reject((error:(t1.errorInfo?.error as?Error, t2.errorInfo?.error as?Error2), isCancelled: isCancelled))
	}
	}.name("\(self.name)-zip(\(task2.name))")
	}

(not merged yet) which was first discussed in #44 .

I personally think we can just keep using variadic Task.all() for API simplicity.
What do you think?