rust-node: avoid meaningless read at the end of odd prefix This should be heavily factored out by the CPU branch predictor anyway. Differential Revision: https://phab.mercurial-scm.org/D8019

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