Add @Override to all the places that need it.

Note that I am not touching or otherwise doing any maintenance on the
code in the Rhino ScriptEngine (com.sun.phobos.script), because my
eventual aim is to switch to using the Nashorn ScriptEngine provided
with Java 8.
Convert JavaHeapObjectVisitor implementations to lambdas.
Merge branch 'master' into language-specific-histograms