[ANN]: io-page 2.0.0, with packaging changes

The io-page library provides page-aligned memory allocation. We use it fairly heavily within the MirageOS unikernel framework.

We have just released io-page 2.0.0 which significantly changes the ocamlfind and opam package structure to remove the optional dependency on mirage-xen-ocaml. Users of io-page who specifically want to use the Xen implementation must now depend on the io-page-xen opam package; while users who specifically want to depend the Unix userspace implementation must now depend on the io-page-unix opam package. To smooth the transition, there are deprecated ocamlfind packages “io-page.unix” and “io-page.xen” which should allow existing applications to compile unmodified, provided the new opam packages are installed.

OPAM repository PR#9516 updated most of the users of io-page to reflect the new structure.If you spot any further breakage, please drop me a note.

Like the previous Cstruct 3.0.0 release, this release also ports the build to use Jbuilder.