codegen: add rhai_mod and submodule support

codegen/ui_tests/rhai_mod_junk_arg.rs

@@ -0,0 +1,29 @@
+use rhai::plugin::*;
+
+#[derive(Clone)]
+struct Point {
+    x: f32,
+    y: f32,
+}
+
+#[export_module]
+pub mod test_module {
+#[rhai_mod("wheeeee")]
+pub mod test_mod {
+pub fn test_fn(input: Point) -> bool {
+    input.x > input.y
+}
+}
+}
+
+fn main() {
+    let n = Point {
+        x: 0.0,
+        y: 10.0,
+    };
+    if test_module::test_fn(n) {
+        println!("yes");
+    } else {
+        println!("no");
+    }
+}