fixing take_raw dropping the externref before resolving it.

[muenchow]

What?

Currently, a #[externref] function that returns a Resource<..> will always return null.
The issue lies in Resource::take_raw: it drops the resource (and frees the slot) before get_externref is called.

This means that by the time get_externref tries to retrieve the ExternRef, the underlying resource has already been dropped.

Fix

The fix is to delay dropping the Resource<..> until the end of the function.
This ensures that get_externref can lookup the ExternRef before the slot is emptied.