rust-nodemap: NodeMap trait with simplest implementation We're defining here only a small part of the immutable methods it will have at the end. This is so we can focus in the following changesets on the needed abstractions for a mutable append-only serializable version. The first implementor exposes the actual lookup algorithm in its simplest form. It will have to be expanded to account for the missing methods, and the special cases related to NULL_NODE. Differential Revision: https://phab.mercurial-scm.org/D7791