Changes

stdlib: provide a sequential interface of the Atomic module from OCaml Multicore

This module provides a purely sequential implementation of the
concurrent atomic references provided by the Multicore OCaml
standard library:

https://github.com/ocaml-multicore/ocaml-multicore/blob/parallel_minor_gc/stdlib/atomic.mli

This sequential implementation is provided in the interest of
compatibility: when people will start writing code to run on
Multicore, it would be nice if their use of Atomic was
backward-compatible with older versions of OCaml without having to
import additional compatibility layers. *)
 (commit: 04d9c42)

fix the testsuite (and add -dno-unique-ids where reasonable)
 (commit: c544d64)

fix the manual and improve the check-stdlib-modules error message
 (commit: 4c05c9d)