Cleanup of directory and existing state handling

[senier]

The fuzzer does not seem to load previous state from the fuzzing directory. Nor does it load state from previous crashes directory.

Clean up the directory handling such that

• An optional state directory file is passed to the fuzzer as separate argument
• Known paths are loaded from state file on startup
• If a state file was configured, the state is regularly saved to that file
• The crashes directory is passed to the fuzzer as separate argument

Ref. #13
Related: #25