Expose public garbage collector API for C language

You can now do epic things like this:

    puts(_gc(xasprintf("%d", 123)));

The _gc() API is shorthand for _defer() which works like Go's keyword:

    const char *s = xasprintf("%d", 123);
    _defer(free, s);
    puts(s);

Be sure to always use -fno-omit-frame-pointer which makes code fast too.

Enjoy! See also #114

tool/viz/printvideo.c

@@ -1351,7 +1351,7 @@ static void OnExit(void) {
 }
 
 static void MakeLatencyLittleLessBad(void) {
-  peekall();
+  _peekall();
   LOGIFNEG1(mlockall(MCL_CURRENT));
   LOGIFNEG1(nice(-5));
 }