Remove need for HostRef<Engine> (#762)

This commit removes the need to use `HostRef<Engine>` in the Rust API.
Usage is retained in the C API in one location, but otherwise `Engine`
can always be used directly.

This is the first step of progress on #708 for the `Engine` type.
Changes here include:

* `Engine` is now `Clone`, and is documented as being cheap. It's not
  intended that cloning an engine creates a deep copy.
* `Engine` is now both `Send` and `Sync`, and asserted to be so.
* Usage of `Engine` in APIs no longer requires or uses `HostRef`.

crates/misc/rust/macro/src/lib.rs

@@ -56,7 +56,7 @@ fn generate_load(item: &syn::ItemTrait) -> syn::Result<TokenStream> {
                 multi_value: true,
                 ..Default::default()
             });
-            let engine = HostRef::new(Engine::new(&config));
+            let engine = Engine::new(&config);
             let store = HostRef::new(Store::new(&engine));
             let global_exports = store.borrow().global_exports().clone();