ZSTD support

It is gaining popularity and it seems that OCaml bindings are abandoned: https://github.com/ygrek/ocaml-zstd 

See format description https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md

And decoding testing tool https://github.com/facebook/zstd/tree/v1.3.4/tests#decodecorpus---tool-to-generate-zstandard-frames-for-decoder-testing