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/build/lib/throw.c

@@ -34,7 +34,7 @@ static bool IsHaltingInitialized(struct Machine *m) {
 
 void HaltMachine(struct Machine *m, int code) {
   CHECK(IsHaltingInitialized(m));
-  gclongjmp(m->onhalt, code);
+  _gclongjmp(m->onhalt, code);
 }
 
 void ThrowDivideError(struct Machine *m) {