Rename `stable_mir` and `rustc_smir`
This PR only renames the two crate names.
There is no doubt that we want to rename `stable_mir` to `rustc_public`, while it hasn't been discussed yet that what the new name for `rustc_smir` should be.
This PR proposes a new name for `rustc_smir`, that is `rustc_public_shim`, since `rustc_smir` now is mostly a proxy to do calls to rustc queries and the public API of rustc that is invoked by the `rustc_public` crate.
However, I don't think that name is good enough. I hope there would be a way better name.
r? `@oli-obk`
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