Add an Instance resolve for the closure function itself

[celinval]

StableMIR doesn't provide an easy to way to retrieve the body of a closure. Instance::resolve_closure will return a shim.

The other possibility would be to allow users to convert ClosureDef to FnDef, but we should make sure that doesn't break any usage of FnDef. @oli-obk, any preference here?

[oli-obk]

If there's no issues with using FnDef, that seems best. From the current set of queries we expose on FnDef, I don't think we'll have any problems