Faster byte and word to hex string

I was alarmed to discover just how slow `String.format()` is for doing
integer to hex conversions. Now that a trace window has been added to
Symon, it became especially clear that I needed a more efficient way to
handle it.

I looked into using `Integer.toHexString()`, but I would have had to
wrap it to do zero-padding, so I decided to just bite the bullet and do
my own with a lookup table. The implementation in `HexUtil` is just as
fast as `Integer.toHexString()`, but also zero-pads appropriately.
Combined with Java's `+` String concatenation, it seems perfectly
adequate.

For yet better performance with the trace window, it would make a lot of
sense to special-case stepping so that it just pops the top line off the
log, and appends to the bottom, rather than re-stringifying the entire
trace log each time. This will be a future enhancement.

src/test/java/com/loomcom/symon/HexUtilTest.java

@@ -0,0 +1,36 @@
+package com.loomcom.symon;
+
+import com.loomcom.symon.util.HexUtil;
+import junit.framework.TestCase;
+
+public class HexUtilTest extends TestCase {
+    public void testByteToHex() {
+        assertEquals("FE", HexUtil.byteToHex(0xfe));
+        assertEquals("00", HexUtil.byteToHex(0));
+        assertEquals("0A", HexUtil.byteToHex(10));
+    }
+
+    public void testByteToHexIgnoresSign() {
+        assertEquals("FF", HexUtil.byteToHex(-1));
+    }
+
+    public void testByteToHexMasksLowByte() {
+        assertEquals("FE", HexUtil.byteToHex(0xfffe));
+        assertEquals("00", HexUtil.byteToHex(0xff00));
+    }
+
+    public void testWordToHex() {
+        assertEquals("0000", HexUtil.wordToHex(0));
+        assertEquals("FFFF", HexUtil.wordToHex(65535));
+        assertEquals("FFFE", HexUtil.wordToHex(65534));
+    }
+
+    public void testWordToHexIgnoresSign() {
+        assertEquals("FFFF", HexUtil.wordToHex(-1));
+    }
+
+    public void testWordToHexMasksTwoLowBytes() {
+        assertEquals("FFFE", HexUtil.wordToHex(0xfffffe));
+        assertEquals("FF00", HexUtil.wordToHex(0xffff00));
+    }
+}