Write demo program

src/bin/demo.rs

@@ -0,0 +1,20 @@
+use deriv::expression::Expression;
+use deriv::eval::Evaluator;
+use deriv::repl::Repl;
+
+struct DummyEvaluator;
+
+impl Evaluator for DummyEvaluator {
+	type Expr = Expression;
+	type Error = std::io::Error;
+
+	fn eval(&mut self, input: Self::Expr) -> Result<Self::Expr, Self::Error> {
+		Ok(input)
+	}
+}
+
+fn main() -> std::io::Result<()> {
+	let mut repl = Repl::new(
+		std::io::stdin(), std::io::stdout(), DummyEvaluator {});
+	repl.run()
+}