Revert "Remove the need for HostRef<Module> (#778)"

This reverts commit 7b33f1c619.

Pushed a few extra commits by accident, so reverting this.

crates/api/tests/import_calling_export.rs

@@ -34,7 +34,7 @@ fn test_import_calling_export() {
 
     let store = Store::default();
     let wasm = wat::parse_str(WAT).unwrap();
-    let module = Module::new(&store, &wasm).expect("failed to create module");
+    let module = HostRef::new(Module::new(&store, &wasm).expect("failed to create module"));
 
     let callback = Rc::new(Callback {
         other: RefCell::new(None),