Rewrite Values#clone for readability and efficiency

[merkste]

The current implementation is a bit cumbersome and potentially inefficient due to resizing of the array lists during adding. This PR resolves this issue and marks the class cloneable.

[kleinj]

Looks good to me. I guess you have tested it in one of your branches? As far as I can tell Values.clone() is never actually used in the main PRISM branch and thus not covered by the test suite.

In the clone() method, you are not simply calling new Values(Values other) on the chance that someone derives a class from Values?

[merkste]

1. Yes, in my local branches and PR Do not unfold the AST on deep-copying #66 actually relies on clone, too.
2. Yes as well, I didn't want to break inheritance. Using a copy constructor in clone-Methods can easily lead to very subtle bugs.