rust-nodemap: generic NodeTreeVisitor This iterator will help avoid code duplication when we'll implement `insert()`, in which we will need to traverse the node tree, and to remember the visited blocks. The structured iterator item will allow different usages from `lookup()` and the upcoming `insert()`. Differential Revision: https://phab.mercurial-scm.org/D7794

rust-nodemap: mutable NodeTree data structure Thanks to the previously indexing abstraction, the only difference in the lookup algorithm is that we don't need the special case for an empty NodeTree any more. We've considered making the mutable root an `Option<Block>`, but that leads to unpleasant checks and `unwrap()` unless we abstract it as typestate patterns (`NodeTree<Immutable>` and `NodeTree<Mutated>`) which seem exaggerated in that case. The initial copy of the root block is a very minor performance penalty, given that it typically occurs just once per transaction. Differential Revision: https://phab.mercurial-scm.org/D7793