Changes

Summary

1. clarified signature; more direct Zstd compression using arrays instead of streams;
2. tuned signature, following isabelle.setup.Environment;
3. tuned signature;
4. tuned signature;
5. tuned signature;
6. clarified signature;
7. prefer new instance, following "make" signature terminology;
8. generic support for XZ and Zstd compression in Isabelle/Scala; support for Zstd compression in Isabelle/ML;
9. clarified signature: default cache is actually dummy and not changed dynamically;
10. clarified Zstd.init(): avoid accidential com.github.luben.zstd.util.Native.load() operation;
11. support for Zstd data compression;