“If the State uses this power systematically in order to force the community to accept a particular sort of money whose employment it desires for reasons of monetary policy, then it is actually carrying through a measure of monetary policy. The States which completed the transition to a gold standard a generation ago, did so from motives of monetary policy. They gave up the silver standard or the credit-money standard because they recognized that the behaviour of the value of silver or of credit money was unsuited to the economic policy they were following.”