Input length extension & some cleanup

[anon8675309]

We wanted driller to be able to extend the length of the input, so it can find more code paths. This patch does that (and a little cleanup such as using proper arguments, even if it is "for internal use only"). There's a little demo program in our blog post on the topic in case you want a test case. https://blog.grimm-co.com/post/guided-fuzzing-with-driller/

[rhelmot]

This looks fine to me. Anyone else?

[salls]

Seems good to me