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

This reverts commit 7b33f1c619.

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

crates/api/src/runtime.rs

@@ -331,7 +331,6 @@ impl Engine {
 /// ocnfiguration (see [`Config`] for more information).
 #[derive(Clone)]
 pub struct Store {
-    // FIXME(#777) should be `Arc` and this type should be thread-safe
     inner: Rc<StoreInner>,
 }