* lisp/emacs-lisp/avl-tree.el: New avl-tree-stack datatype. Add new

* lisp/emacs-lisp/avl-tree.el: New avl-tree-stack datatype.  Add new
traversal functions for avl-trees.
(avl-tree--stack): New struct.
(avl-tree-stack-p, avl-tree--stack-repopulate): New funs.
(avl-tree-enter): Add optional `updatefun' arg.
(avl-tree--do-enter): Add optional `updatefun' arg.  Change return value.
(avl-tree-delete): Add optional `test' and `nilflag' args.
(avl-tree--do-delete): Add `test' and `nilflag' args.  Change return value.
(avl-tree-member): Add optional `nilflag'
(avl-tree-member-p): New function.
(avl-tree-mapc, avl-tree-mapf, avl-tree-mapcar): New functions.
(avl-tree-stack, avl-tree-stack-pop, avl-tree-stack-first)
(avl-tree-stack-empty-p): New functions.