Avoid size calculation for subclass list

Getting this accurate is not important, but most weak collections
will attempt to get an accurate count at increased expense. This
patch just saves the last list size and uses that for future list
allocations.

Author: headius

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

@@ -1019,14 +1019,9 @@ protected void setModuleSuperClass(RubyClass superClass) {
 
     @JRubyMethod
     public IRubyObject subclasses(ThreadContext context) {
-        Map<RubyClass, Object> subclasses = this.subclasses;
-        int sizeEstimate = subclasses == null ? 0 : subclasses.size();
-
-        if (sizeEstimate == 0) {
-            return RubyArray.newEmptyArray(context.runtime);
-        }
+        int subclassEstimate = this.subclassEstimate;
 
-        RubyArray<RubyClass> subs = RubyArray.newArray(context.runtime, sizeEstimate);
+        RubyArray<RubyClass> subs = RubyArray.newArray(context.runtime, subclassEstimate == -1 ? 4 : subclassEstimate);
 
         concreteSubclasses(subs);
 
@@ -3093,6 +3088,7 @@ public IRubyObject invokeFrom(ThreadContext context, CallType callType, IRubyObj
     private ObjectAllocator allocator; // the default allocator
     protected ObjectMarshal marshal;
     private volatile Map<RubyClass, Object> subclasses;
+    private int subclassEstimate = -1;
     public static final int CS_IDX_INITIALIZE = 0;
     public enum CS_NAMES {
         INITIALIZE("initialize");