Improve the Cargo dependency situation

[helgoboss]

The current situation is this: Since AFAIK there are no transitive dependencies in Cargo, if someone wants to use reaper-medium (medium-level API), they also have to depend on reaper-low. And not just that. It must be exactly the version that reaper-medium uses. The situation gets worse if one wants to use reaper-high.

I don't know what's the perfect way to handle this in Rust. I've seen that some crates re-export stuff. E.g. reaper-medium could re-export some things from reaper-low. But one thing I really don't like about this is that suddenly for one symbol there are two ways to address that symbol. The same symbol is available in 2 crates. That's very confusing.

There's an RFC that might help: rust-lang/rust#44663

If anyone has an idea, please let me know.