## Context
Some native libraries may want to explicitly delete an externref object without
waiting for the module instance to be deleted.
In addition, it may want to add a cleanup function.
## Proposed Changes
Implement:
* `wasm_externref_objdel` to explicitly delete an externeref'd object.
* `wasm_externref_set_cleanup` to set a cleanup function that is called when
the externref'd object is deleted.
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