retire ocplib-endian. Since OCaml 4.01 the primitives are part of Bigarray

[hannesm]

in the same spirit as inhabitedtype/faraday#24 (thanks @seliopou) we can directly use the primitives provided by the OCaml runtime (since 4.01, https://caml.inria.fr/mantis/view.php?id=5771 -- cstruct is only available on >= 4.02.3).

Why? Well, less dependencies are always better. ocplib-endian is additionally LGPL licensed (see https://github.com/OCamlPro/ocplib-endian/blob/master/COPYING.txt) - thus pretty deep down in our dependency chain a not very permissively licensed library.

What do we loose? big endian support (which AFAICT hasn't been used anyways, or has it?) -- it can be re-added by either some preprocessing (cpp0?) or by dynamic checks for Sys.big_endian. I don't know enough about cpp0 do go that way, but I can add the dynamic Sys.big_endian checks if this is desired.

EDIT: I added Sys.big_endian tests, performance could be improved using a preprocessor (but since ocplib as well uses runtime checks, there's no performance regression in here)

[hannesm]

locally I observed breakage in git, fixed in mirage/ocaml-git#233

[avsm]

lgtm; i've started the git constraining PR to prepare this for release. I believe that Sys.big_endian can be inlined if it's sufficiently at the toplevel that the inliner picks it up. It might be worth experimenting with the conditional into a toplevel value and looking at the lambda output to see if it's still there after compilation.

[hannesm]

is there anything I can do to move this PR forward?