Basilar Artery Trunk Aneurysm: Unruptured Saccular Aneurysm of the Distal Basilar Artery Trunk at the Origin of the Left Superior Cerebellar Artery with Brainstem Compression, LEO Stent-Induced Flow Diversion of the Aneurysm with Resolution of the Mass Effect and No Perforator Occlusion, and Good Clinical Outcome with Midterm Follow-Up