Parallelize execution

We generate one benchmark run per combination of serializer, compressor and encryptor ([see code](https://github.com/ascoderu/compression-benchmarks/blob/21b9866b2272e43ef85cf8a58dcc17146025a29d/benchmarks/__main__.py#L75)). The runs have no dependency on each other, so it should be possible to run them in parallel using [multiprocessing](https://docs.python.org/3/library/multiprocessing.html).