lj_auditlog: Add "event" object

Log both "events" (new) and "memory" (already had this.)

The "event" and "memory" log messages are complementary. First we log
objects in memory, as detailed context, and then we log events, which
can refer to those objects.

For example, when a trace stops (i.e. is successfully recorded and
assembled), then we first log the GCtrace object in memory:

    {
        "type": "memory",
        "hint": "GCtrace",
        "address": 139883362546248,
        "data": <bin of size 120>
    }

and then later we log a "trace_stop" event that refers to this memory:

    {
        "type": "event",
        "event": "trace_stop",
        "GCtrace": 139883362546248
    }

So the "memory" messages are effectively providing context for later
"events".

src/lj_auditlog.c

@@ -42,17 +42,15 @@ static void bin_32(void *ptr, int n) {
 /* -- low-level object logging API ---------------------------------------- */
 
 /* Ensure that the log file is open. */
-static void ensure_log_open()
-{
+static void ensure_log_open() {
   if (!fp) {
     fp = fopen("audit.log", "w");
     lua_assert(fp != NULL);
   }
 }
 
 /* Log a snapshot of an object in memory. */
-static void log(const char *type, void *ptr, unsigned int size)
-{
+static void log_mem(const char *type, void *ptr, unsigned int size) {
   ensure_log_open();
   fixmap(4);
   str_16("type");    /* = */ str_16("memory");
@@ -61,16 +59,24 @@ static void log(const char *type, void *ptr, unsigned int size)
   str_16("data");    /* = */ bin_32(ptr, size);
 }
 
+static void log_event(const char *type, int nattributes) {
+  lua_assert(nattributes <= 253);
+  fixmap(nattributes+2);
+  str_16("type");  /* = */ str_16("event");
+  str_16("event"); /* = */ str_16(type);
+  /* Caller fills in the further nattributes... */
+}
+
 /* -- high-level LuaJIT object logging ------------------------------------ */
 
 /* Log a trace that has just been compiled. */
 void lj_auditlog_trace_stop(jit_State *J, GCtrace *T)
 {
-  /* Log the memory containing the GCtrace object and other important
-     memory that it references. */
-  log("GCtrace", T, sizeof(*T));
-  log("MCode[]", T->mcode, T->szmcode);
-  log("SnapShot[]", T->snap, T->nsnap * sizeof(*T->snap));
-  log("SnapEntry[]", T->snapmap, T->nsnapmap * sizeof(*T->snapmap));
+  log_mem("GCtrace", T, sizeof(*T));
+  log_mem("MCode[]", T->mcode, T->szmcode);
+  log_mem("SnapShot[]", T->snap, T->nsnap * sizeof(*T->snap));
+  log_mem("SnapEntry[]", T->snapmap, T->nsnapmap * sizeof(*T->snapmap));
+  log_event("trace_stop", 1);
+  str_16("GCtrace"); /* = */ uint_64((uint64_t)T);
 }