Kernel#printf supports two call signatures:
printf(String, *args)
printf(IO, String, *args)
We want to identify the String argument, which is the format string.
Previously we would return the 0th and 1st arguments, which gives some
FPs when the 1st arg is not a format string.
We now try to rule out the trivial case by checking if arg 0 has a
string value, and then assuming it is the format string. Otherwise we
fall back to returning both arguments.
This still has some false positive potential, but less than previously.
