facebook/zstd to submodule

[keewon]

Thanks for writing zstd-erlang.
I wanted to use latest zstd, but it seems that this repository contains zstd 1.3.3 itself.
This PR changes priv/zstd to git submodule.

How I verified

$ git submodule update --init
$ rebar g-d
$ rebar co
$ erl -pa ebin
1> Compressed = zstd:compress(<<"Hello, World!">>).
<<40,181,47,253,32,13,105,0,0,72,101,108,108,111,44,32,87,
  111,114,108,100,33>>
2> zstd:decompress(Compressed).
<<"Hello, World!">>

How I added this one to my Elixir project (mix.exs)

{:zstd, github: "keewon/zstd-erlang", branch: "zstd_to_submodule", submodules: true},