Implement Serialize/Deserialize for Hashed<V, S>

[TehPers]

Objective

This allows users to serialize and deserialize both prehashed values and maps/sets with prehashed keys, avoiding the need for a conversion.

Solution

When serializing, only the inner stored value (of type V) is serialized. The struct acts as a transparent wrapper around it, so the Hashed struct is invisible to serde.

When deserializing, the reverse happens: the inner value is deserialized, then the Hashed struct is initialized from the deserialized value, computing the hash at that time. This means that the computed hash may differ between what was serialized and what was deserialized if the hasher produces two different values at those times - for example, across different runs of the application using different versions of bevy. This should normally not matter since the hash usually only needs to be consistent in the same run of the application.

Testing

There are unit tests added for both Hashed<V, FixedHasher> and HashMap<Hashed<K, FixedHasher>, V, PreHash>. The tests for the hashmap are to ensure that PreHashMap can be serialized/deserialized as well, and serializes in the expected format.

Some other bevy crates depend on serde_test, but that crate appears to be unmaintained now, so I'm using serde_json for testing instead.