Create a `MutMirVisitor`

[celinval]

We currently provide a MirVisitor that allows users to visit the MIR of a function body, but without mutating it. It would be super helpful to provide a new version that allows body mutation.

The mutation would only be applied to the StableMIR body. But that can be quite handy if tools want to apply some transformations before running any analysis. For example, in Kani we instrument the code with extra checks before the analysis.

[makai410]

@rustbot claim