Turn Event into a tagged union

If we ever need arbitrary data or more than very few bytes on `Events` we just
have to add a `void *` field in the `data` union.

src/os/event.c

@@ -12,7 +12,7 @@
 #include "memory.h"
 #include "misc2.h"
 
-// event.data will be cleaned up after the event is processed
+// event will be cleaned up after it gets processed
 #define _destroy_event(x)  // do nothing
 KLIST_INIT(Event, Event, _destroy_event)
 

src/os/event.h

@@ -10,7 +10,9 @@ typedef enum {
 
 typedef struct {
   EventType type;
-  void *data;
+  union {
+    int signum;
+  } data;
 } Event;
 
 void event_init(void);

src/os/signal.c

@@ -69,9 +69,7 @@ void signal_accept_deadly()
 
 void signal_handle(Event event)
 {
-  int signum = *(int *)event.data;
-
-  free(event.data);
+  int signum = event.data.signum;
 
   switch (signum) {
     case SIGINT:
@@ -157,8 +155,9 @@ static void signal_cb(uv_signal_t *handle, int signum)
 
   Event event = {
     .type = kEventSignal,
-    .data = xmalloc(sizeof(int))
+    .data = {
+      .signum = signum
+    }
   };
-  *(int *)event.data = signum;
   event_push(event);
 }