D5370: rust: core implementation of missingancestors (no bindings)

[Yuya Nishihara]

>   @yuja: do you mean one of those Differential Revisions of this system for each commit, sure I can do.

I don't know how Phabricator is working, but probably yes. I meant we want
D5370, D5371, and D5372 instead of a folded D5370. I think phabsend will
do that way.

>   With respect to rust-cpython bindings, I'm currently waiting for feedback on https://github.com/dgrunwald/rust-cpython/issues/164
>   Perhaps you'd have an idea ?

No, but it looks interesting. I'll take a look this weekend.