Add Snappy compression

[meme]

To do:

• Remove config.h and define macros using the setup_compression.py script
• Documentation
• Tests

[ThomasWaldmann]

Thanks for the PR!

Usually there is some discussion before adding bigger features like this, do we have a ticket about this and did you read #1633?

[ThomasWaldmann]

BTW, there are some compile issues on travis-ci, see there.

[meme]

Took a look at the issue you mentioned and I do not believe that Snappy meets all the requirements. I'll keep working on this one but it doesn't belong in Borg.

[ThomasWaldmann]

Sorry about the work you invested. Just open an issue before starting major work next time.

Yesterday, I looked around and my impression was that snappy is somehow between zstd and lz4, but being quite close, so I didn't see a real advantage of adding it, considering we already have lz4 and zstd.

Having more code (bundled or as a dependency) always means more burden that needs to be rectified.

[meme]

No worries about the work! The codebase is well factored so not too much time was spent at all. I hope to make more contributions to Borg in the future and will make sure to scope them out in an issue beforehand.

---

If I may ask: could this PR be labelled with hacktoberfest-accepted? And perhaps you may want to mark the repository with a tag for hacktoberfest as the event is now opt-in (more details here).

[ThomasWaldmann]

Done, thanks for the hints.