Add code generation for doubly linked list

codegen/dbl_list/snippets/node_to_list

@@ -0,0 +1,15 @@
+  {T}List output = {{.first = node, .last = node, .total_size = node->string->size, .node_count = 1}};
+
+  while (output.first->prev != NULL) {{
+    output.total_size += output.first->prev->string->size;
+    output.first = output.first->prev;
+    ++(output.node_count);
+  }}
+
+  while (output.last->next != NULL) {{
+    output.total_size += output.last->next->string->size;
+    output.last = output.last->next;
+    ++(output.node_count);
+  }}
+
+  return output;