Generators for the other Joe-E array types

Tyler Close writes:
>I was thinking PowerlessArray would provide a public implementation of
>the Generator interface and make use of Java's support for covariant
>return types in the implementation of the snapshot() method, so...
>
>    public final class
>    Generator<T> implements org.joe_e.array.Generator<T> {
>        public PowerlessArray<T>
>        snapshot() { return ...; }
>    }

Got it.  That should do the trick nicely.