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.

compiler/stable_mir/src/lib.rs

@@ -39,6 +39,7 @@ pub mod visitor;
 
 pub use error::*;
 use mir::mono::Instance;
+use ty::{FnDef, GenericArgs};
 
 /// Use String for now but we should replace it.
 pub type Symbol = String;
@@ -233,6 +234,9 @@ pub trait Context {
 
     /// Item requires monomorphization.
     fn requires_monomorphization(&self, def_id: DefId) -> bool;
+
+    /// Resolve an instance from the given function definition and generic arguments.
+    fn resolve_instance(&mut self, def: FnDef, args: &GenericArgs) -> Option<Instance>;
 }
 
 // A thread local variable that stores a pointer to the tables mapping between TyCtxt