[WIP] Expose trace map to Python code

[jwilk]

This allows Python code to manually poke at the trace map, which might be useful for blackbox testing. Possible alternative to #9.

Before this gets merged:

• [ ] Document the new feature.
• [ ] Discuss addition of a similar API to afl-clang-fast on afl-users.*

^{* Last time I innovated with new API, AFL proper adapted it under a different (better!) name, and then I had to rename mine… Let's not make this mistake again.}

[jwilk]

Discuss addition of a similar API to afl-clang-fast on afl-users

https://groups.google.com/d/msg/afl-users/Hv57XTZ-sVw/HEIhZiwJEQAJ