Add support to `InstanceDef` and monomorphized `Instance`

[celinval]

We should be able to create a stable version of Instance since we now have GenericArgs. This will help us work with monomorphized code.

[oli-obk]

Where do we get it from? Or do you mean for creating manually and then getting the MIR for it?

[celinval]

Where do we get it from? Or do you mean for creating manually and then getting the MIR for it?

Yes. In Kani we usually start traversing from non-generic functions and momomorphize as needed.

I also think it's ok if for now we expose a conversion from internal to stable.