rust-nodemap: building blocks for nodetree structures This is similar to `nodetreenode` in `revlog.c`. We give it a higher level feeling for ease of handling in Rust context and provide tools for tests and debugging. The encoding choice is dictated by our ultimate goal in this series, that is to make an append-only persistent version of `nodetree`: the 0th Block must be adressed from other Blocks. Differential Revision: https://phab.mercurial-scm.org/D7787