Change clone method of collections to consistently return iso^.

[jemc]

This PR changes the clone method of Array, List, HashMap, and HashSet to return iso^.

This is a more flexible return type, allowing the calling code to take advantage of the fact that the return value is indeed the only reference to the new object that exists so far. This is also more consistent the clone method signature for String and Flags, which already do this.

[andymcn]

Maybe I'm being dumb, but surely this isn't capability safe.

Array.clone() creates a shallow copy. So if you have an array of Foo refs then the new clone array will contain references to the same Foo objects, so the array itself is not isolated.

For String and Flags it's OK, because the contents is copied too.

[jemc]

You're right.

In my case it is a List of vals, so it's okay, but in other cases like the ones you mentioned, it won't be. For now, I guess I have to maintain my own method in the package that clones this kind of list and gives an iso^ return, unless there is some kind of guard for this that we could use in the standard library (under a different method name) that lets you clone a collection to iso^ if all the contents are of a capability such that it would be okay.