Add a conversion method for going from smir::Ty to rustc::Ty

[oli-obk]

In order to be able to invoke some rustc operations (like Instance::resolve), we need the rustc version of GenericArgs, which contains Ty, Lifetime and Const. We should add functionality in the rustc_smir crate for converting back to these three rustc types when starting out with a smir type. This will mostly involve writing the inverse of their Stable::stable method.

### Tasks
- [ ] Implement `RustcInternal` for `Lifetime`

[celinval]

• Ty / GenericArgs: Add stable Instance::body() and RustcInternal trait rust#116964
• Const: Remove fold code and add Const::internal() to StableMIR rust#117113

[celinval]

BTW, for Lifetime, we currently return re_erased which was added here: rust-lang/rust#117688. However, we should implement a better mechanism without any loss of information.