Initial Use Case: Port MiniRust

[celinval]

@RalfJung suggested that the MiniRust translation crate (minimize) is a good candidate for us to assess the impact of migrating to StableMIR.

This issue tracks the work needed to port MiniRust. I believe we can migrate a few modules at a time:

Tasks

Beta Give feedback

1. Update MiniRust a newer nightly version
2. Migrate minimize::ty module
3. Migrate minimize::rvalue module
4. Migrate the rest of the code

[RalfJung]

Update MiniRust a newer nightly version

That part is done now, I wanted to do this anyway.

[celinval]

I created minirust/minirust#166 to migrate rvalue.