afl: 2.10b -> 2.23b

Looks like mostly performance enhancements and stability fixes.  The main
user facing changes appear to be:

- The -Z option was removed
- A macro named FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION is defined when
    compiling with afl-gcc

Full changelog at http://lcamtuf.coredump.cx/afl/ChangeLog.txt