By Alek Arend Baker Mayfield’s stock with the Carolina Panthers appears to be rising. What initially began as a real quarterback competition within the team has now become Mayfield’s to lose. The veteran quarterback is competing against Sam Darnold and rookie Matt Corral for the starting position. Mayfield appears to be running away with it. Panthers head coach Matt Rhule has nothing but praise for his new quarterback after a week-plus of training camp. “He’s definitely improving as he gets more and more of a feel for what he can do at the line of scrimmage. . . . What he’s done in 10 days is …