Added tests + fixed bunch of bugs in GC implementation

runtime/virt_stack.c

@@ -0,0 +1,45 @@
+#include "virt_stack.h"
+#include <malloc.h>
+
+virt_stack *vstack_create() {
+    return malloc(sizeof (virt_stack));
+}
+
+void vstack_destruct(virt_stack *st) {
+    free(st);
+}
+
+void vstack_init(virt_stack *st) {
+    st->cur = RUNTIME_VSTACK_SIZE;
+    st->buf[st->cur] = 0;
+}
+
+void vstack_push(virt_stack *st, size_t value) {
+    if (st->cur == 0) {
+        assert(0);
+    }
+    --st->cur;
+    st->buf[st->cur] = value;
+}
+
+size_t vstack_pop(virt_stack *st) {
+    if (st->cur == RUNTIME_VSTACK_SIZE) {
+        assert(0);
+    }
+    size_t value = st->buf[st->cur];
+    ++st->cur;
+    return value;
+}
+
+void* vstack_top(virt_stack *st) {
+    return  st->buf + st->cur;
+}
+
+size_t vstack_size(virt_stack *st) {
+    return RUNTIME_VSTACK_SIZE - st->cur;
+}
+
+size_t vstack_kth_from_start(virt_stack *st, size_t k) {
+    assert(vstack_size(st) > k);
+    return st->buf[RUNTIME_VSTACK_SIZE - 1 - k];
+}