make ref_unlink optional and itcm pass a simple test again

note: there's no marking yet

tests/compiler/gc/itcm/trace.ts

@@ -0,0 +1,26 @@
+@global const GC_TRACE = true;
+
+import "allocator/arena";
+import "collector/itcm";
+
+import { HEADER_SIZE } from "runtime";
+
+assert(HEADER_SIZE == 16);
+assert(gc.implemented);
+
+gc.collect(); // trigger init
+
+class Ref {}
+
+trace("# ref = new Ref()");
+var ref = new Ref();
+trace("# arr = new Array(1)");
+var arr = new Array<Ref | null>(1);
+trace("# arr[0] = ref");
+arr[0] = ref;
+trace("# arr[0] = null");
+arr[0] = null;
+
+// TODO...
+
+@start export function main(): void {}