Chris Willman Senior Music Writer and Chief Music Critic What the world needs now is Burt, sweet Burt, and there’s a bereft feeling to be had in knowing we won’t be getting any more of the haunting, mind-twisting melodies that characterized the composer’s work over a period of more than six decades of sumptuous pop. But Burt Bacharach left a gift behind — something he actually held the finished product of in his hands, before he died Feb. 8 at age 94: “The Songs of Bacharach & Costello,” a deluxe boxed set of the material he worked on with Elvis Costello from the mid-’90s on up till nearly the present day. As anthologized by Costello with Steve Berkowitz (who also helps assemble Bob Dylan’s “Bootleg Series” sets), the new collection is a worthy testament to a true maestro who was doing some of the best work of his life in his final decades.