xz: add multi-thread support

[surban]

Adds XzEncoder::parallel and XzDecoder::parallel methods that enable multi-threaded compression and decompression.

Care must be taken not that call flush() on the encoder in quick succession, as this would severely impact multi-threaded compression performance.

Gated by the xz-parallel crate feature.

[NobodyXu]

Thanks, just some minor change to API

[NobodyXu]

Thanks LGTM, @robjtede can you have a double check please?

[robjtede]

cool 👍🏻

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (8f9394d) to head (a75df3c).
Report is 5 commits behind head on main.

Additional details and impacted files

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