Ensure that serializers injectable into Drupal\Core\KeyValueStore are aware of PHP objects

✨ Make serializer customizable for Cache\DatabaseBackend RTBC landed!

As of form state, it probably needs some prefix for cache keys to make serialization swappable

The igbinary nodule can read existing cache entries created by the php serializer to allow a transition to igbinary. But it doesn't work the other way.
Maybe a clearing all caches in case the igbinary module gets uninstalled is the better way compared to a prefix.
This should also happen when you move your site to a system where igbinary is not available.

The KV store is not a cache, caches are fixed already

There was suggestion to copy bin on module install