Update / replace protobuf exercise

See #2502 and in particular, from @randomPoison:
> Oh that's great. As much as I like the protobuf exercise, it is pretty complicated and I think students spend a lot of time just wrapping their heads around what they need to do. This seems way more concise and understandable, and better demonstrates the usage of lifetimes.
>
> I think both of these are nice improvements over our current ones, so I think these would be great to integrate :)