* Add mutated tracker on resources and ChangedRes query for added or mutated resources.
* ResMut:::new() now takes a reference to a 'mutated' flag in its archetype.
* Change FetchResource so that get() returns an Option. Systems using Resources will only be called if all fetched Resources are Some(). This is done to implement ChangedRes, which is Some iff the Resource has been changed.
* Add OrRes for a logical or in tuples of Resource queries.
* Separate resource query get() in is_some() and get() methods for clarity
* Remove unneeded unsafe
* Change ResMut::new()
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