Skip wrapping if function returns Dynamicc.

codegen/ui_tests/return_mut_ref.stderr

@@ -1,4 +1,4 @@
-error: cannot return a reference to Rhai
+error: Rhai functions cannot return references
   --> $DIR/return_mut_ref.rs:12:38
    |
 12 | pub fn test_fn(input: &mut Clonable) -> &mut bool {

codegen/ui_tests/return_pointer.stderr

@@ -1,4 +1,4 @@
-error: cannot return a pointer to Rhai
+error: Rhai functions cannot return pointers
   --> $DIR/return_pointer.rs:12:33
    |
 12 | pub fn test_fn(input: Clonable) -> *const str {

codegen/ui_tests/return_shared_ref.stderr

@@ -1,4 +1,4 @@
-error: cannot return a reference to Rhai
+error: Rhai functions cannot return pointers
   --> $DIR/return_shared_ref.rs:12:33
    |
 12 | pub fn test_fn(input: Clonable) -> &'static str {