rust: add a ObjectID struct

rust: add a ObjectID struct

We'd like to be able to write some Rust code that can work with object
IDs.  Add a structure here that's identical to struct object_id in C,
for easy use in sharing across the FFI boundary.  We will use this
structure in several places in hot paths, such as index-pack or
pack-objects when converting between algorithms, so prioritize efficient
interchange over a more idiomatic Rust approach.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>