remove 'final' from AST, Indexer

External users sometimes want to extend the AST with their own nodes or a node
that is “this existing node with a slight tweak.” Having methods marked `final`
forces them into forking Rumur just to remove these annotations. Recent analysis
has also suggested there is no significant performance benefit from `final`.¹

¹ https://16bpp.net/blog/post/the-performance-impact-of-cpp-final-keyword/