libafl basic havoc + token mutator

This uses the libafl StdScheduledMutator with havoc_mutations and token_mutations.

Make sure to have cargo installed and just type make to build.

Run with:

AFL_CUSTOM_MUTATOR_LIBRARY=custom_mutators/libafl_base/libafl_base.so AFL_CUSTOM_MUTATOR_ONLY=1 afl-fuzz ...