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