Generic.Collections: Add `operator in`, Find(), Min() and Max() in TArrayHelper<>

Summary

I think the issue title speaks for itself, but I’ll explain anyway, just in case:

• operator in - self-explanatory.
• Find() - returns index of the element passed by const/constref if it's found in the array, or -1 otherwise.
• Min() - performs linear lookup for the minimum element in the array. Has a default argument for value to return if the array is empty.
• Max() - same as Min(), but searches for the maximum element.