Rollup merge of #112600 - celinval:stable-mir-rvalue, r=oli-obk

Introduce a `Stable` trait to translate MIR to SMIR

This PR introduces a trait `Stable` which defines a type `T` and a `stable()` method to convert the current type to its stable MIR version.

This change is just an implementation detail, and I wanted to get some feedback to whether it would be cleaner than having the `rustc_<type>_to_<type>()` methods for every type we translate to SMIR.

r? `@oli-obk`
r? `@spastorino`

Author: matthiaskrgr