When printing hex numbers, we were printing the wrong thing sometimes. This
was because we were dividing the digit to print by 15 instead of 16. Also,
dividing by 16 is the same as shifting four bits to the right, which is a
bit closer to our actual intention in this case, so let's use a shift
instead.
The printf formatting mini-language actually allows you
to pass a '*' character in place of the fill width specification,
in which case it eats one of the passed in arguments and uses it
as width, so implement that.
It's kinda funny how I can make a mistake like this in Serenity and then
get so used to it by spending lots of time using this API that I start to
believe that this is how printf() always worked..
