How to make ABI compatible between crates.io release and rustc_smir

[celinval]

In order to release StableMIR, we need to ensure compatibility between the compiler data structures and the published crate.

[celinval]

@oli-obk raised an important concern:

I have a concern with the crates.io release. It will either break (because we ship the Context impl in it, and thus the usage of the non SMIR rustc APIs), or it doesn't use any non-SMIR APIs, but then the data structures are incompatible with the ones shipped by rustc

[celinval]

I documented the current proposal here: https://hackmd.io/jBRkZLqAQL2EVgwIIeNMHg