Lazy initialize volatile fields

Eagerly initializing these adds memory churn to the allocation of
many RubyModule types that will never even access these fields.
Instead we lazily initialize them using an atomic update.

core/src/main/java/org/jruby/IncludedModuleWrapper.java

@@ -69,6 +69,7 @@ public IncludedModuleWrapper(Ruby runtime, RubyClass superClass, RubyModule orig
     public IncludedModuleWrapper(Ruby runtime, RubyClass superClass, RubyModule origin, RubyModule methodsHolder) {
         super(runtime, superClass, origin);
         origin.addIncludingHierarchy(this);
+        // force writeable methods table so we see future updates
         methods = methodsHolder.getMethodsForWrite();
     }
 

core/src/main/java/org/jruby/PrependedModule.java

@@ -60,8 +60,10 @@ public PrependedModule(Ruby runtime, RubyClass superClass, RubyModule prependedC
             classIndex(superClass.getClassIndex()); // use same ClassIndex as metaclass, since we're technically still of that type
         }
         this.methods = prependedClass.methods;
-        prependedClass.methods = Collections.EMPTY_MAP;
+        prependedClass.methods = null;
         prependedClass.methodLocation = this;
+
+        Map<String, DynamicMethod> methods = getMethods();
         for (Map.Entry<String, DynamicMethod> entry : methods.entrySet()) {
             DynamicMethod method = entry.getValue();
             if (moveRefinedMethod(entry.getKey(), method, prependedClass)) {

core/src/main/java/org/jruby/RubyClass.java

@@ -1415,7 +1415,7 @@ void addInvalidatorsAndFlush(InvalidatorList invalidators) {
         invalidators.add(methodInvalidator);
 
         // if we're not at boot time, don't bother fully clearing caches
-        if (!runtime.isBootingCore()) cachedMethods.clear();
+        if (!runtime.isBootingCore()) getCachedMethods().clear();
 
         getSubclassesForRead().forEachClass(invalidators);
     }