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
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