Update to O'Caml 3.12.0, add test target.

Some of the highlights in release 3.12 are:

  * Polymorphic recursion is supported, using explicit type
    declarations on the recursively-defined identifiers.
  * First-class modules: module expressions can be embedded as
    values of the core language, then manipulated like any other
    first-class value, then projected back to the module level.
  * New operator to modify a signature a posteriori: S with type
    t := tau denotes signature S where the t type component is
    removed and substituted by the type tau elsewhere.
  * New notations for record expressions and record patterns:
    { lbl } as shorthand for { lbl = lbl }, and { ...; _ } marks
    record patterns where some labels were intentionally omitted.
  * Local open let open ... in ... now supported by popular demand.
  * Type variables can be bound as type parameters to functions;
    such types are treated like abstract types within the
    function body, and like type variables (possibly generalized)
    outside.
  * The module type of construct enables to recover the module
    type of a given module.
  * Explicit method override using the method! keyword, with
    associated warnings and errors.

(asau)