Return a table enumerator (cursor) that can be used to
enumerate all key-value pairs using PL_advance_hash_table_enum().
The enumerator must be discarded using PL_free_hash_table_enum().
It is safe for another thread to add symbols while the table is being
enumerated, but undefined whether or not these new symbols are visible.
If another thread deletes a key that is not yet enumerated it will not