Equality quickfix

Delegate cstruct equality to a layer below.
[Opened](https://github.com/mirage/ocaml-cstruct/issues/24).

Did you mean to do it in C, or more something akin to [this](https://github.com/mirleft/ocaml-nocrypto/blob/fd0f94ff4bb2d9007a2541fa542bf7849c4dce48/src/common.ml#L71)?