gc: Speed up new_object by adding new blocks to the front of the list

This way we don't have to traverse the whole list to the very end every
time we want a new object.

src/gc.c

@@ -124,17 +124,17 @@ new_block(struct gc *gc)
 static struct gc_object *
 new_object_inner(struct gc *gc)
 {
-    struct gc_block **cur = &gc->block;
+    struct gc_block *cur = gc->block;
 
-    while (*cur != NULL) {
-        struct gc_block *block = *cur;
+    while (cur != NULL) {
+        struct gc_block *block = cur;
         for (size_t i = 0; i < GC_OBJECTS_PER_BLOCK; i++) {
             if (block->objects[i].status == GC_STATUS_FREE) {
                 return &block->objects[i];
             }
         }
 
-        cur = &block->next;
+        cur = block->next;
     }
 
     struct gc_block *nb = new_block(gc);
@@ -142,7 +142,8 @@ new_object_inner(struct gc *gc)
         return NULL;
     }
 
-    *cur = nb;
+    nb->next = gc->block;
+    gc->block = nb;
 
     return &nb->objects[0];
 }