Prep the way for type casting.

src/ir/from_syntax.rs

@@ -104,6 +104,8 @@ impl syntax::Expression {
                 (vec![], ValueOrRef::Ref(loc, ArcIntern::new(name)))
             }
 
+            syntax::Expression::Cast(_, _, _) => unimplemented!(),
+
             // Primitive expressions are where we do the real work.
             syntax::Expression::Primitive(loc, prim, mut expressions) => {
                 // generate a fresh new name for the binding site we're going to