hash_storage: fix assignment operator (#211)

The assignment operator of hash_storage now calls the `clear` function
to ensure proper behavior. Previously, it only cleared the destination
hash_storage if the source hash_storage was empty, leading to keys
being left in the destination hash_storage that did not exist in the
source, and not being copied if they already exist in the destination
hash_storage. Now, the assignment operator clears the destination
hash_storage every time, preventing this issue.

Co-authored-by: VirtualDeep <[email protected]>

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