Separate the IR evaluator, add an interesting stack-breaking case.

build.rs

@@ -77,7 +77,8 @@ fn generate_tests(f: &mut File, path_so_far: PathBuf) -> std::io::Result<()> {
                     f,
                     "  let ir = syntax.type_infer().expect(\"example is typed correctly\");"
                 )?;
-                writeln!(f, "  let ir_result = ir.eval();")?;
+                writeln!(f, "  let ir_evaluator = crate::ir::Evaluator::default();")?;
+                writeln!(f, "  let ir_result = ir_evaluator.eval(ir.clone());")?;
                 writeln!(f, "  match (&syntax_result, &ir_result) {{")?;
                 writeln!(f, "     (Err(e1), Err(e2)) => assert_eq!(e1, e2),")?;
                 writeln!(f, "     (Ok((v1, o1)), Ok((v2, o2))) => {{")?;