The field ordering here is actually significant because of the drop
order: we want to drop the artifact before dropping the engine.

The reason for this is that dropping the Artifact will de-register the
trap handling metadata from the global registry. This must be done before
the code memory for the artifact is freed (which happens when the store
is dropped) since there is a chance that this memory could be reused by
another module which will try to register its own trap information.

Note that in Rust, the drop order for struct fields is from top to
bottom: the opposite of C++.

In the future, this code should be refactored to properly describe the
ownership of the code and its metadata.

Fixes #2434