Fix and extend ControlFlow `traverse_inorder` example
Fix and extend ControlFlow `traverse_inorder` example

1. The existing example compiles on its own, but any usage fails
   to be monomorphised and so doesn't compile. Fix that by using
   a mutable reference as an input argument.
2. Added an example usage of `traverse_inorder` showing how we
   can terminate the traversal early.

Fixes #90063

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust