Remove fold code and add Const::internal()

We are not planning to support user generated constant in the
foreseeable future, so we are removing the Fold logic for now in
favor of the Instance::resolve logic.

The Instance::resolve was however incomplete, since we weren't handling
internalizing constants yet. Thus, I added that.

I decided to keep the Const fields private in case we decide to
translate them lazily.

compiler/stable_mir/src/visitor.rs

@@ -47,12 +47,12 @@ impl Visitable for Const {
         visitor.visit_const(self)
     }
     fn super_visit<V: Visitor>(&self, visitor: &mut V) -> ControlFlow<V::Break> {
-        match &self.literal {
+        match &self.kind() {
             super::ty::ConstantKind::Allocated(alloc) => alloc.visit(visitor)?,
             super::ty::ConstantKind::Unevaluated(uv) => uv.visit(visitor)?,
             super::ty::ConstantKind::Param(_) => {}
         }
-        self.ty.visit(visitor)
+        self.ty().visit(visitor)
     }
 }