Split no_stdlib and no_std into two features.

examples/no_std.rs

@@ -0,0 +1,13 @@
+#![cfg_attr(feature = "no_std", no_std)]
+
+use rhai::{Engine, EvalAltResult};
+
+fn main() -> Result<(), EvalAltResult> {
+    let mut engine = Engine::new();
+
+    let result = engine.eval::<i64>("40 + 2")?;
+
+    assert_eq!(result, 42);
+
+    Ok(())
+}