some docs and speed up value copying

core/src/main/java/org/jruby/runtime/Binding.java

@@ -317,10 +317,15 @@ public BacktraceElement getBacktrace() {
                     new Frame(),
                     Visibility.PUBLIC);
 
+    /**
+     * Duplicate this binding and setup the proper cloned instance of the eval scope so that any previously
+     * captured variables still exist but are not shared with the original binding.
+     * @param context the current thread context
+     * @return the duplicated binding
+     */
     public Binding dup(ThreadContext context) {
         Binding newBinding = new Binding(this);
-        DynamicScope scope = getEvalScope(context.runtime);
-        newBinding.evalScope = scope.dup();
+        newBinding.evalScope = getEvalScope(context.runtime).dupEvalScope();
         return newBinding;
     }
 }

core/src/main/java/org/jruby/runtime/DynamicScope.java

@@ -600,12 +600,13 @@ public DynamicScope cloneScope() {
         }
     }
 
-    public DynamicScope dup() {
-        DynamicScope newScope = new ManyVarsDynamicScope(staticScope.duplicate(), parent);
-        IRubyObject[] values = getValues();
-        for (int i = 0; i < values.length; i++) {
-            newScope.setValueDepthZero(values[i], i);
-        }
+    /**
+     * Binding needs to clone its scope with all the current values.
+     * @return a duplicate of this scope with all the current values
+     */
+    public DynamicScope dupEvalScope() {
+        ManyVarsDynamicScope newScope = new ManyVarsDynamicScope(staticScope.duplicate(), parent);
+        newScope.setVariableValues(getValues());
         return newScope;
     }
 }

core/src/main/java/org/jruby/runtime/scope/ManyVarsDynamicScope.java

@@ -50,6 +50,13 @@ private void allocate() {
         if(variableValues == null) variableValues = IRubyObject.array(staticScope.getNumberOfVariables());
     }
 
+    // WARNING:  This is used when dup'ing an eval scope.  We know the size and that it will 100% always be
+    // a ManyVarsDynamicScope.  This should not be used by anything else.  If there ever ends up being another
+    // use then it should be documented in this warning.
+    public void setVariableValues(IRubyObject[] variableValues) {
+        this.variableValues = variableValues;
+    }
+
     public IRubyObject[] getValues() {
         return variableValues;
     }