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/isheap.c

@@ -26,7 +26,7 @@
  * @assume stack addresses are always greater than heap addresses
  * @assume stack memory isn't stored beneath %rsp (-mno-red-zone)
  */
-bool isheap(void *p) {
+bool _isheap(void *p) {
   int x, i;
   uintptr_t rsp;
   asm("mov\t%%rsp,%0" : "=r"(rsp));