Aggregate all string literals during instrumentation

Hi,

as we discussed yesterday on my talk about [fuzzing in Python](https://www.youtube.com/watch?v=s7r0iooKGHY) it may be good if Atheris extracted all string literals to use them later during mutating of an input.

Apparently Atheris currently extract literals that are directly compared with a variable like this: `if x == "abc"` but it doesn't extract literals that are used in other ways like: `if x.startswith("some string"`.

On the other hand, one downside to extracting all string literals are logging string formatting messages which may not be that useful for fuzzing. But I am not sure how big of a problem that is, and, maybe there should be an option to inspect the extracted strings and influence them.