We want to keep StableMIR definitions and logic separate from any sort of conversion and usage of internal rustc code. So we bundle all unstable items that have no stability guarantees into `stable_mir::unstable`.
add a new trait `InternalCx`, which defines the methods that are fine to call from `RustcInternal`. `RustcInternal::internal()` then takes a `impl InternalCx<'tcx>` instead of `TyCtxt<'tcx>`.
make `tcx` in `SmirCtxt` public, since we need to pass it to `RustcInternal::internal()` in `SmirInterface`.
note that this commit delete `convert/error.rs`, we would use `SmirError::from_internal` instead.
**Unresolved questions:**
- There are still a few direct calls to rustc's internals scattered across `impl Stable`s, but most of them appear to be relatively stable, e.g., `mir::interpret::ConstAllocation::inner(self)` and `mir::syntax::SwitchTargets::otherwise(self)`.
- rewrite all `SmirInterface` apis.
- add `BridgeTys` to impl those associated types in `Bridge`.
- move `**_def()` stuffs living in `impl Tables` from `rustc_internal` to `stable_mir`.
- With `Context` wrapped by `SmirInterface`, the stable-mir's TLV stores a pointer to `SmirInterface`, while the rustc-specific TLV stores a pointer to tables.
- This PR make the `rustc_smir` mod public.
