HandyKey, by the developer Hikaru, allows use of the stock keyboard to type one-handed in portrait orientation. It accomplishes this by shrinking the keyboard slightly and moving it down to the left or right side. It can also be used in "full" mode which centers the keyboard. You can switch easily among the three modes by swiping to the left or right with two fingers. The "extra" keys on the 6 and 6 Plus keyboards can also be disabled if you wish.