A decade ago, pianist Herbie Hancock established his “New Standards” initiative, aiming to wed sophisticated improvisation to a contemporary American pop songbook (post-Berlin, Gershwin, Porter, et al). At last, after several disastrous attempts, he’s justified such a project with River: The Joni Letters — infusing well-known high art pop songs by inimitable Joni Mitchell with the depth of lyrical, inspired jazz.