StableMIR: Proof-of-concept implementation + test
This PR is part of the [project Stable MIR](https://github.com/rust-lang/project-stable-mir). The PR deletes old re-exports from rustc_smir and introduces a proof-of-concept implementation for APIs to retrieve crate information.
The implementation follows the [design described here](https://hackmd.io/XhnYHKKuR6-LChhobvlT-g?view), but instead of using separate crates for the implementation, it uses separate modules inside `rustc_smir`.
The API introduced at this point should be seen just as an example on how we are planning to structure the communication between tools and the compiler.
I have not explored yet what should be the right granularity, the best starting point for users, neither the best way to implement it.
r? ``````@oli-obk``````
