Merge orc11 into gpfsl repo

Now that we have the technology to have multiple opam packages in one repo, there is IMO no good reason to keep orc11 and gpfsl in different repos. The diamonds their dependencies with std++ form is already annoying enough (and that is not entirely resolved, but at least now the graph of repo dependencies has no diamond any more).

I did some git magic to import the orc11 commit history as well.

@haidang any objections? I'd like to merge this ASAP since it will bitrot when anything changes in either repo. Cc @robbertkrebbers