Change default backend to `zlib-rs` instead

[Xuanwo]

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Hi, I'm starting this thread to discuss whether it would be a good idea to switch the default backend to zlib-rs, as it outperforms both zlib-ng and miniz_oxide.

Or at the very least, we can mention this in our README and revise any statements referring to a purely Rust backend.

I'm willing to perform this change.