Could the num_cpus dependency be replaced by std::thread::available_parallelism?

From looking at the implementation of both and the discussion on https://github.com/seanmonstar/num_cpus I get the impression that this would _not_ be a breaking change.

Start by locating the num_cpus dependency and its usage in the repository, then compare that implementation with Rust's std::thread::available_parallelism. Run the existing test suite before and after the dependency change; done means the dependency is replaced without breaking supported behavior.

Written by the indexing model from the issue text.