[wasmtime-api] reduce examples complexity: hostref for store/engine (#489)

* reduce amount of store.clone()

* use HostRef Engine as ref / use Engine::default()

wasmtime-api/examples/hello.rs

@@ -21,8 +21,8 @@ impl Callable for HelloCallback {
 fn main() -> Result<()> {
     // Initialize.
     println!("Initializing...");
-    let engine = HostRef::new(Engine::new(Config::default()));
-    let store = HostRef::new(Store::new(engine));
+    let engine = HostRef::new(Engine::default());
+    let store = HostRef::new(Store::new(&engine));
 
     // Load binary.
     println!("Loading binary...");
@@ -30,19 +30,18 @@ fn main() -> Result<()> {
 
     // Compile.
     println!("Compiling module...");
-    let module =
-        HostRef::new(Module::new(store.clone(), &binary).context("> Error compiling module!")?);
+    let module = HostRef::new(Module::new(&store, &binary).context("> Error compiling module!")?);
 
     // Create external print functions.
     println!("Creating callback...");
     let hello_type = FuncType::new(Box::new([]), Box::new([]));
-    let hello_func = HostRef::new(Func::new(store.clone(), hello_type, Rc::new(HelloCallback)));
+    let hello_func = HostRef::new(Func::new(&store, hello_type, Rc::new(HelloCallback)));
 
     // Instantiate.
     println!("Instantiating module...");
     let imports = vec![hello_func.into()];
     let instance = HostRef::new(
-        Instance::new(store.clone(), module, imports.as_slice())
+        Instance::new(&store, &module, imports.as_slice())
             .context("> Error instantiating module!")?,
     );