Reimplement the event queue in event.c using klist.h

- Add a new macro to klist.h: kl_empty()

   The whole point of abstract data structures is to avoid reimplementing
   common actions. The emptiness test seems to be such an action.

 - Add a new function attribute to func_attr.h: FUNC_ATTR_UNUSED

   Some of the many functions created by the macros in klist.h may end up not
   being used. Unused functions cause compilation errors as we compile with
   -Werror. To mark those functions as possibly unused we can use the
   FUNC_ATTR_UNUSED now.

 - Pass `Event` by value

   `Event` is such a small struct that I don't think we should allocate heap space
   and pass it by reference. Let's use the stack and memory cache in our favor
   passing it by value.

src/func_attr.h

@@ -34,6 +34,7 @@
   #define FUNC_ATTR_CONST __attribute__((const))
   #define FUNC_ATTR_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
   #define FUNC_ATTR_ALWAYS_INLINE __attribute__((always_inline))
+  #define FUNC_ATTR_UNUSED __attribute__((unused))
 
   #ifdef __clang__
     // clang only
@@ -90,6 +91,10 @@
   #define FUNC_ATTR_ALWAYS_INLINE
 #endif
 
+#ifndef FUNC_ATTR_UNUSED
+  #define FUNC_ATTR_UNUSED
+#endif
+
 #ifndef FUNC_ATTR_NONNULL_ALL
   #define FUNC_ATTR_NONNULL_ALL
 #endif