Allow mutating a module-qualified function's first argument if it is a variable.

tests/modules.rs

@@ -73,6 +73,10 @@ fn test_module_resolver() -> Result<(), Box<EvalAltResult>> {
     module.set_fn_4("sum".to_string(), |x: INT, y: INT, z: INT, w: INT| {
         Ok(x + y + z + w)
     });
+    module.set_fn_1_mut("double".to_string(), |x: &mut INT| {
+        *x *= 2;
+        Ok(())
+    });
 
     resolver.insert("hello", module);
 
@@ -90,6 +94,18 @@ fn test_module_resolver() -> Result<(), Box<EvalAltResult>> {
         42
     );
 
+    assert_eq!(
+        engine.eval::<INT>(
+            r#"
+                import "hello" as h;
+                let x = 21;
+                h::double(x);
+                x
+            "#
+        )?,
+        42
+    );
+
     #[cfg(not(feature = "unchecked"))]
     {
         engine.set_max_modules(5);