Fix no_std build.

examples/no_std.rs

@@ -2,6 +2,12 @@
 
 use rhai::{Engine, EvalAltResult, INT};
 
+#[cfg(feature = "no_std")]
+extern crate alloc;
+
+#[cfg(feature = "no_std")]
+use alloc::boxed::Box;
+
 fn main() -> Result<(), Box<EvalAltResult>> {
     let engine = Engine::new();