“broad-based tax cut . . . accommodated by a program of open market purchases . . . would almost certainly be an effective stimulant to consumption.... A money-financed tax cut is essentially equivalent to Milton Friedman’s famous “helicopter drop” of money.... Of course . . . the government could . . . even acquire existing real or financial assets. If . . . the Fed then purchased an equal amount of Treasury debt with newly created money, the whole operation would be the economic equivalent of direct open market operations in private assets.”