gix-mailmap: Derive std::hash::Hash for Snapshot

[caio]

Hello!

I'd like to cache output that relies on mailmaps.

Using the object id or a hash of the raw bytes is an alternative but the format allows for comments and whatnot. At present, the best alternative I think is newtyping and implementing Hash through Snapshot::iter(). Having hash on Snapshot lets me avoid that.

[Byron]

Thanks a lot! Makes sense.