Implements turn assistance using keypress interval
When the user requests a turn by hitting the left or right arrow key, the game will check every frame between the key press and KEY_PRESS_INTERVAL_SECS to determine if a turn is possible. This relieves the burden on the player to hit the button at *exactly* the right time. Perhaps this could be coupled with another constant, but for now it seems like KEY_PRESS_INTERVAL_SECS is working pretty well.