compare_by_identity: remove alloc for empty Hash

For non-empty Hashes, this function needs to rehash all the stored values (using the new `compare` and `hash` functions from `identhash`). It does so by writing into a newly allocated `tmp` Hash, and then transferring ownership of its st table into `self`.

For empty Hashes, we can skip allocating this `tmp`, because there's nothing to re-hash. We can just modify our new st table's `type` in-place.

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