rust-re2: add wrapper for calling Re2 from Rust This assumes that Re2 is installed following Google's guide. I am not sure how we want to integrate it in the project, but I think a follow-up patch would be more appropriate for such work. As it stands, *not* having Re2 installed results in a compilation error, which is a problem as it breaks install compatibility. Hence, this is gated behind a non-default `with-re2` compilation feature. Differential Revision: https://phab.mercurial-scm.org/D7910