Switch to private Relations store

[MGatner]

Relations should not be stashed in $attributes because this can create some unintended issues with model methods and casting. Instead they will be moved to a private stash of $relations which is checked before re-loading related items.

This will constitute a breaking change and come with the next major version.

[MGatner]

Consider using a WeakReference or even a centralized WeakMap to make these more leak-safe.