Adress review comments from https://github.com/ziglang/zig/pull/13977
by using the same naming convention as zstd.
And by using `finish()` instead of `close()` for the finalisation of the compressed stream.
rationale:
- it is not the same as how close() is usually used, since it must be called to flush and write the final bytes. And as such it may fail.
- it is not the same `flush` in the deflate code, which allows to keep writting more bytes later, and doesn't write the final checksum.
- it is the same name as used in the original zlib library (Z_FINISH)
Also, use a packed struct for the header, which seems a better fit.
67b3e07260