Enforce signatures on user get/set/index functions

codegen/ui_tests/rhai_fn_index_setter_multiple.stderr

@@ -0,0 +1,11 @@
+error: conflicting index_set
+  --> $DIR/rhai_fn_index_setter_multiple.rs:12:40
+   |
+12 |     #[rhai_fn(name = "foo", index_set, index_set)]
+   |                                        ^^^^^^^^^
+
+error[E0433]: failed to resolve: use of undeclared type or module `test_module`
+  --> $DIR/rhai_fn_index_setter_multiple.rs:23:8
+   |
+23 |     if test_module::test_fn(n) {
+   |        ^^^^^^^^^^^ use of undeclared type or module `test_module`