=act #16330 deprecating akka.util.Crypt

 * Fix for #16330 
 * Additionally clean several files up: final modifiers on objects and final class members, unused imports, iterators.nonEmpty instead of !iterators.isEmpty
thanks for looking it up! so seems in this case it's fine. 
Longer story is - `nonEmpty` is not on all types, sometimes it's slapped onto a type by an implicit, just by looking at this line I was unsure which case we're taking about. Since The PR is more about `Crypt.scala`, I'd rather want to minimise the surface we thouch :)