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

libc/runtime/free_s.c

@@ -26,5 +26,7 @@ void free_s(void *v) {
   void **pp = (void **)v;
   void *p = NULL;
   lockxchg(pp, &p);
-  if (isheap(p)) weakfree(p);
+  if (_isheap(p)) {
+    _weakfree(p);
+  }
 }