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/main/java/com/loomcom/symon/Simulator.java

@@ -247,6 +247,7 @@ public class Simulator implements Observer {
         // Spin up the new run loop
         runLoop = new RunLoop();
         runLoop.start();
+        traceLog.simulatorDidStart();
     }
 
     private void handleStop() {
@@ -268,6 +269,7 @@ public class Simulator implements Observer {
                 statusPane.updateState(cpu);
             }
         });
+        traceLog.simulatorDidStop();
         if (traceLog.isVisible()) {
             traceLog.refresh();
         }
@@ -291,6 +293,8 @@ public class Simulator implements Observer {
             ram.fill(0x00);
             // Clear the console.
             console.reset();
+            // Reset the trace log.
+            traceLog.reset();
             // Update status.
             SwingUtilities.invokeLater(new Runnable() {
                 public void run() {
@@ -320,13 +324,9 @@ public class Simulator implements Observer {
      * Perform a single step of the simulated system.
      */
     private void step() throws MemoryAccessException {
-
         cpu.step();
-        // TODO: We need to profile this for performance. Possibly allow
-        // a flag to turn trace on/off
-        synchronized(traceLog) {
-            traceLog.append(cpu.getCpuState());
-        }
+
+        traceLog.append(cpu.getCpuState());
 
         // Read from the ACIA and immediately update the console if there's
         // output ready.