Sovereign immunity — Foreign States — Libel actions against Soviet press agencies — Whether Foreign Sovereign Immunities Act 1976 should be given retroactive effect to apply to cases commenced before its effective date — Whether agencies were “foreign States” entitled to claim immunity under the Act — Effect of Soviet Ambassador's certifications that they were, respectively, an organ of the Soviet State and an agency or instrumentality thereof — Whether agencies entitled to immunity under the Act for their activities in publishing allegedly defamatory articles in periodicals distributed in the United States — The law of the United States