Support for Content-Encoding value zstd in Rack::Deflater.

This algorithm is currently only supported by Chrome, Edge and Firefox. Safari has "positive" as their standard position, but I could see no implementation work.

Useful for things such as JSON payloads and other dynamic content that cannot be compressed ahead of time. The compression ratio is similar to gzip, but it's twice as fast to decompress and slightly quicker to compress too.

Resources

• https://caniuse.com/zstd
• https://lemire.me/blog/2021/06/30/compressing-json-gzip-vs-zstd/