Proposal: #ReadabilityAddedByLayerCreator

[mark.reinhold at oracle.com]

Issue summary
-------------

  #ReadabilityAddedByLayerCreator --- Provide a means by which the code
  that creates a layer can add readability edges from the modules in that
  layer to other modules, whether those modules are in that layer or in
  other layers. [1]

Proposal
--------

Revise `java.lang.reflect.Layer` so that the multi-parent layer-creation
methods added for #NonHierarchicalLayers [2] return a capability object
rather than the resulting layer.  The capability will be represented by
instances of a new nested class, `java.lang.reflect.Layer.Controller`,
which will define an `addReads` method and also a method to retrieve the
actual layer:

    public final static class Controller {

        /** Return the actual layer */
        public Layer layer();

        /**
         * Add the specified reads edge, if not already present.
         * Return this controller, so that invocations can be
         * chained.
         */
        public Controller addReads(Module source, Module target);

    }

Typical usage would be:

    Layer.Controller lc = Layer.defineModules(cf, parents, clf);
    Module m1 = lc.layer().findModule("m1").get();
    Module m2 = otherLayer.findModule("m2").get();
    lc.addReads(m1, m2);    // Ensures m1.canRead(m2) == true

The source module must be in the controller's layer; the target module
can be in any layer.  This method does nothing if the source module can
already read the target module.

Like all capability objects, instances of `Layer.Controller` must be
handled with care since leaking them could enable malicious use.


[1] http://openjdk.java.net/projects/jigsaw/spec/issues/#ReadabilityAddedByLayerCreator
[2] http://mail.openjdk.java.net/pipermail/jpms-spec-experts/2016-October/000442.html