LONDON/REYKJAVIK — Iceland’s Prime Minister Sigmundur David Gunnlaugsson is to resign, his party said on Tuesday, becoming the first casualty of leaked documents from a Panamanian law firm which have shone a spotlight on the finances of an array of politicians and public figures worldwide. The Panama Papers showed the premier’s wife owned an offshore company with big claims on Icelandic banks, infuriating many in his country who joined mass street protests calling for him to resign. The more than 11.5 million documents, leaked from the Panamanian law firm Mossack Fonseca, have caused public outrage over how the world’s rich and powerful are able to stash their cash and avoid taxes while many people suffer austerity and hardship.