allocator: rework alloc_delete

Added a switch case at start of alloc_delete to only run it on
container (read: heap allocated) types.

If adding to free vector, iterate through free vector first to ensure
we've not added it already.  Reset the object before adding it, so
reuse is trivial.

src/allocator.c

@@ -158,8 +158,35 @@ end:
 
 void alloc_delete(alloc_t *alloc, lisp_t *lisp)
 {
+  switch (tag_get(lisp))
+  {
+  case TAG_CONS:
+  case TAG_VEC:
+  case TAG_STR:
+    break;
+  case TAG_NIL: // These can't be deleted (not allocated)
+  case TAG_SMI:
+  case TAG_SYM:
+  default:
+    FAIL("Unreachable");
+    return;
+  }
+
   alloc_node_t *node = lisp_to_node(lisp);
   assert(node && node->metadata.references == 0);
+
+  // If already present in the free vector, stop.
+  FOR_VEC(i, &alloc->free_vec, alloc_node_t *)
+  {
+    alloc_node_t *other = VEC_GET(&alloc->pages, i, alloc_node_t *);
+    if (other == node)
+    {
+      return;
+    }
+  }
+
+  // Otherwise, add to the free vector.
+  lisp_reset(lisp);
   vec_append(&alloc->free_vec, &node, sizeof(node));
 }
 
@@ -179,7 +206,7 @@ void alloc_free(alloc_t *alloc)
   FOR_VEC(i, &alloc->pages, page_t *)
   {
     page_t *page = VEC_GET(&alloc->pages, i, page_t *);
-    // Iterate through every alloc_node in this page
+    // Iterate through every alloc_node in this page (dynamic walk)
     for (u64 j = 0; j < VEC_SIZE(&page->data, u8);)
     {
       alloc_node_t *node = (alloc_node_t *)(vec_data(&page->data) + j);