Rework Kaleidoscope.use to be a compile-time recursive function

Based on suggestions from Wez Furlong (@wez) in #135, this replaces the
`Kaleidoscope.use` function with one that does its thing at compile time.

The net result is that we save a considerable amount of code, while still having
all of the benefits, and being 100% backwards compatible, no code needs to
change.

We may want to adjust existing code to use `Kaleidoscope.use` directly, and drop
any trailing NULLs we may have had. But there is no rush to do so.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

src/Kaleidoscope.cpp

@@ -34,19 +34,6 @@ Kaleidoscope_::loop(void) {
   }
 }
 
-void
-Kaleidoscope_::use(KaleidoscopePlugin *plugin, ...) {
-  va_list ap;
-  KaleidoscopePlugin *p;
-
-  plugin->begin();
-  va_start(ap, plugin);
-  while ((p = va_arg(ap, KaleidoscopePlugin*)) != NULL) {
-    p->begin();
-  }
-  va_end(ap);
-}
-
 void
 Kaleidoscope_::replaceEventHandlerHook(eventHandlerHook oldHook, eventHandlerHook newHook) {
   for (byte i = 0; i < HOOK_MAX; i++) {

src/Kaleidoscope.h

@@ -33,23 +33,7 @@ extern HARDWARE_IMPLEMENTATION KeyboardHardware;
 
 #define KEYMAP_SIZE (sizeof(keymaps) / ROWS / COLS / sizeof(Key))
 
-/*
- * The `USE_PLUGINS()` macro is a clever hack, to make it seem like
- * `Kaleidoscope.use()` is type-safe. It pushes its arguments into an
- * appropriately typed array, so anything that does not fit the criteria, will
- * trigger a compiler error.
- *
- * It then never uses the array, and passes the plugins over to
- * `Kaleidoscope.use`, adding the trailing `NULL`, making it even easier to use.
- *
- * Since the array this macro creates is never used, the compiler will optimize
- * it out fully. As such, by using this macro, we incur neither any size
- * penalties, nor any run-time penalties. Everything happens at compile-time.
- */
-#define USE_PLUGINS(plugins...) ({                              \
-          static KaleidoscopePlugin *__p[] = {plugins, NULL};   \
-          Kaleidoscope.use(plugins, NULL);                      \
-        })
+#define USE_PLUGINS(plugins...) Kaleidoscope.use(plugins)
 
 class KaleidoscopePlugin {
  public:
@@ -65,7 +49,34 @@ class Kaleidoscope_ {
   }
   void setup(void);
   void loop(void);
-  void use(KaleidoscopePlugin *plugin, ...) __attribute__((sentinel));
+
+  // ---- Kaleidoscope.use() ----
+
+  // First, we have the zero-argument version, which will satisfy the tail case.
+  inline void use() {
+  }
+
+  // Then, the one-argument version, that gives us type safety for a single
+  // plugin.
+  inline void use(KaleidoscopePlugin *p) {
+    p->begin();
+  }
+
+  // We have a no-op with a int argument, as a temporary hack until we remove
+  // the last instance of a NULL-terminated Kaleidoscope.use() call.
+  inline void use(int) {
+  }
+
+  // And the magic is in the last one, a template. The first parameter is
+  // matched out by the compiler, and passed to one of the functions above. The
+  // rest of the parameter pack (which may be an empty set in a recursive case),
+  // are passed back to either ourselves, or the zero-argument version a few
+  // lines above.
+  template <typename... Plugins>
+  void use(KaleidoscopePlugin *first, Plugins&&... plugins) {
+    use(first);
+    use(plugins...);
+  }
 
   // ---- hooks ----