Add stable Instance::body() and RustcInternal trait
The `Instance::body()` returns a monomorphized body.
For that, we had to implement visitor that monomorphize types and constants. We are also introducing the RustcInternal trait that will allow us to convert back from Stable to Internal.
Note that this trait is not yet visible for our users as it depends on Tables. We should probably add a new trait that can be exposed.
The tests here are very simple, and I'm planning on creating more exhaustive tests in the project-mir repo. But I was hoping to get some feedback here first.
r? ```@oli-obk```
The `Instance::body()` returns a monomorphized body.
For that, we had to implement visitor that monomorphize types and
constants. We are also introducing the RustcInternal trait that will
allow us to convert back from Stable to Internal.
Note that this trait is not yet visible for our users as it depends on
Tables. We should probably add a new trait that can be exposed.
Also add a few methods to instantiate instances and get an instance
definition.
We're still missing support to actually monomorphize the instance body.
In smir use `FxIndexMap` to store indexed ids
Previously we used `vec` for storing indexed types, which is fine for small cases but will lead to huge performance issues when we use `smir` for real world cases.
Addresses https://github.com/rust-lang/project-stable-mir/issues/35
r? ``@oli-obk``
