Change StorageMemoryUsage type from struct to enum

[notinmybackyaard]

@leudz
As mentioned earlier, I attempted to convert the StorageMemoryUsage type into a trait. The first approach involved defining an Out type for each Storage type that implements the StorageMemoryUsage trait, allowing the memory_usage function to return an Option<Box> type. The second approach was to add StorageMemoryUsage as an associated trait to the Storage trait. The first approach felt overly verbose, and the second required too extensive code changes, so I opted for a simpler solution by handling it as an enum to enable more detailed information during debugging. If there are any suggested modifications, please let me know.

[leudz]

Thanks for putting this together!
Sadly an enum cannot work. Users are free to add other storages that could be any type ('static + Storage is the minimum bound). This is done with the CustomStorageAccess trait.
Sorry.

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What do you think of something like this?

pub trait MemoryUsage {
    type Out;

    fn detailed_memory_usage(&self) -> Self::Out;
}

SparseSet and Entities could implement this trait. Maybe in the future World and AllStorages too. For Unique it would have to be implemented on the views but I wouldn't bother (at least for now) since its size is fixed.
StorageMemoryUsage wouldn't change.
MemoryUsage would be public so users can call detailed_memory_usage directly.

Does this make sense? Do you think it would also be too verbose?