I realized that libbz2-rs-sys exists, by the same folks who make libz-rs-sys - it's a Rust reimplementation of libbz2. This means we can do that same thing as in #5562, and avoid having to cross-compile C. I then realized after I removed the feature flag that we weren't actually running bz2 regrtests in CI, because we weren't passing it as a feature to cargo build, and so something like half of the tests in test_bz2.py were failing. So, I more or less finished up the impl, by making some code in zlib.rs generic over the specific Decompress struct.