Uncommon.Cs: revise with current cstruct in mind (opam depends on cstruct 3.0.0)

[hannesm]

• Since 1.7.0, append and concat are part of cstruct.
• Since 2.2.0, create zeroes out the buffer, create_unsafe does not.

[pqwy]

Github conflict resolution. Not even once.

https://github.com/mirleft/ocaml-nocrypto/commit/79e2ffdebbfbe35f32e9be734233000b086975b6