Small fixes (warnings) and adding 'crc32' commandline tool

[sgothel]

Pardon my potentially convoluted pull request, Stephan.

Was testing your excellent crc32 library on arm32 and arm64 and added a commandline tool.

On the way, I removed some warnings and also played with the std::async multithreading, which I am not comfy with at this point yet.

Thank you very much!