Add itertools module.

[tillahoffmann]

This PR adds a module implementing the functionality of Python's itertools package. I originally implemented this a few years ago to run machine learning experiments, mostly to generate cartesian products of learning rate schedule, optimizer, etc.

@jjmaestro recently suggested it would be useful to have this functionality available in Skylib.

The implementation here is WIP as it is missing markdown files in the docs directory. I thought I'd get your feedback before getting into writing docs beyond docstrings.

[jjmaestro]

Could one of the reviewers have a look? 🙏 Thanks!