rust: add safe bindings to bdiff.c I wrote C FFI bindings manually rather than using a bindgen build step because there are only 2 structs and 3 functions and they're not going to change. Note that the relative path in build.rs means that cargo publish will no longer work. If in the future we want to publish to crates.io, we would probably need to add a Makefile step that copies bdiff sources into the hg-core crate.