Rearranged and commented Layer.on() and Layer.off()

The boolean wasOn was unnecessary, and there was no need to call
bitSet() (or bitClear(), in the case of Layer.off()) if the test
passed. Mostly, I just added a few explanatory comments.

(Aslo reversed the sense of the on/off test in Layer.on() and .off())

@algernon likes it better this way, and I agree.

src/layers.cpp

@@ -136,30 +136,40 @@ void Layer_::on(uint8_t layer) {
   if (layer_count != 0 && layer >= layer_count)
     return;
 
-  bool wasOn = isOn(layer);
+  // If the target layer was already on, return
+  if (isOn(layer))
+    return;
 
+  // Otherwise, turn on its bit in LayerState
   bitSet(LayerState, layer);
+
+  // If the target layer is above the previous highest active layer,
+  // update highestLayer
   if (layer > highestLayer)
     updateHighestLayer();
 
-  /* If the layer did turn on, update the keymap cache. See layers.h for an
+  // Update the keymap cache (but not liveCompositeKeymap; that gets
-   * explanation about the caches we have. */
+  // updated separately, when keys toggle on or off. See layers.h)
-  if (!wasOn)
+  updateActiveLayers();
-    updateActiveLayers();
 }
 
 // Deactivate a given layer
 void Layer_::off(uint8_t layer) {
-  bool wasOn = isOn(layer);
+  // If the target layer was already off, return
+  if (!bitRead(LayerState, layer))
+    return;
 
+  // Turn off its bit in LayerState
   bitClear(LayerState, layer);
+
+  // If the target layer was the previous highest active layer,
+  // update highestLayer
   if (layer == highestLayer)
     updateHighestLayer();
 
-  /* If the layer did turn off, update the keymap cache. See layers.h for an
+  // Update the keymap cache (but not liveCompositeKeymap; that gets
-   * explanation about the caches we have. */
+  // updated separately, when keys toggle on or off. See layers.h)
-  if (wasOn)
+  updateActiveLayers();
-    updateActiveLayers();
 }
 
 boolean Layer_::isOn(uint8_t layer) {