rust-nodemap: abstracting the indexing In the forthcoming mutable implementation, we'll have to visit node trees that are more complex than a single slice, although the algorithm will still be expressed in simple indexing terms. We still refrain using `#[inline]` indications as being premature optimizations, but we strongly hope the compiler will indeed inline most of the glue. Differential Revision: https://phab.mercurial-scm.org/D7792

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