rust-revlog: a trait for the revlog index
As explained in the doc comment, this is the minimum needed
for our immediate concern, which is to implement a nodemap
in Rust.
The trait will be later implemented in `hg-cpython` by the
index Python object implemented in C, thanks to exposition
of the corresponding functions as a capsule.
The `None` return cases in `node()` match what the `index_node()`
C function does.
Differential Revision: https://phab.mercurial-scm.org/D7789
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh