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

This reverts commit 7b33f1c619.

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

crates/api/examples/memory.rs

@@ -86,7 +86,7 @@ fn main() -> Result<(), Error> {
 
     // Compile.
     println!("Compiling module...");
-    let module = Module::new(&store, &binary).context("> Error compiling module!")?;
+    let module = HostRef::new(Module::new(&store, &binary).context("> Error compiling module!")?);
 
     // Instantiate.
     println!("Instantiating module...");