Willie Dixon is the man who changed the style of the blues. As a songwriter and producer, the man was a genius. If you wanted a hit song, you went to Willie Dixon, played it like he said play it, and sing it like he said sing it, and you damn near always had a hit.    Willie Dixon taught bass players how to rock `n' roll. Listen to him on Chuck Berry's Chess recordings of "Rock and Roll Music" and "Heelin' and Rockin'". He took big band music and Mississippi blues and melded them into something new, opening the door for Motown and others to walk in and take it even further.     Dixon: "The blues are the roots and the other musics are the fruits. It's better keeping the roots alive because it means better fruits from now on. The blues will always be, because the blues are the roots of all American music. As long as American music survives, so will the blues."