Initial GC integration (#196)

tests/compiler/std/gc-basics.ts

@@ -0,0 +1,34 @@
+import "allocator/arena";
+import "collector/itcm";
+
+// a class to test with
+class MyObject {
+  a: u32;
+}
+function MyObject_visit(ref: usize): void {} // function table index == classId ?
+
+// allocate a managed instance
+var obj: MyObject | null = changetype<MyObject>(__gc_allocate(offsetof<MyObject>(), MyObject_visit));
+obj.a = 123;
+
+// check header
+{
+  let head = changetype<usize>(obj) - 16;
+  let next = load<u32>(head, 0) & ~3;
+  let prev = load<u32>(head, 4);
+  assert(next != 0 && prev != 0 && next == prev);
+  let visitFn = load<u32>(head, 8);
+  assert(visitFn == changetype<u32>(MyObject_visit));
+  let unused = load<u32>(head, 12);
+  assert(unused == 0);
+  let a = load<u32>(head, 16);
+  assert(a == 123);
+}
+
+gc.collect(); // should keep 'obj' because it's a referenced root (see trace output)
+obj = null;
+gc.collect(); // should free 'obj' because it isn't referenced anymore (see trace output)
+
+var obj2: MyObject; // should also iterate globals defined late
+
+export function main(): i32 { return 0; }