Internal FHashMap improvements; better union operation.

The hash codes of node keys are now cached, improving performance,
particularly on `get`s.

`FMap.union` now accepts an optional value combiner, making it more
general.

Author: Scott L. Burson

Makefile

@@ -5,7 +5,7 @@ test:
 	java com/ergy/fset/TestSuite 10000 >& test.out
 
 doc:
-	javadoc -link http://docs.oracle.com/javase/7/docs/api/ -d Doc/JavaDoc com/ergy/fset/{F,AbstractF}*.java
+	javadoc -link http://docs.oracle.com/javase/7/docs/api/ -d Doc/JavaDoc com/ergy/fset/{F,AbstractF,BinaryOp}*.java
 
 fset.jar: all com/ergy/fset/*.class
 	jar cf fset.jar com/ergy/fset/*.class

com/ergy/fset/BinaryOp.java

@@ -0,0 +1,19 @@
+/*
+ * BinaryOp.java
+ *
+ * Copyright (c) 2013, 2014 Scott L. Burson.
+ *
+ * This file is licensed under the Library GNU Public License (LGPL), v. 2.1.
+ */
+
+
+package com.ergy.fset;
+
+/**
+ * A binary operation of type (T, T) -> T.
+ */
+public interface BinaryOp<T> {
+
+	T apply(T x, T y);
+
+}