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/testlib/benchrunner.c

@@ -51,7 +51,7 @@ void testlib_benchwarmup(void) {
  * @see BENCH()
  */
 void testlib_runallbenchmarks(void) {
-  peekall();
+  _peekall();
   mlockall(MCL_CURRENT);
   nice(-1);
   __log_level = kLogWarn;