Relax Arc wrapping requirement

[AaronFeickert]

Both Statement and Witness require that certain fields be wrapped in Arc, under the assumption that the caller is likely to reuse data like parameters and input sets that are expensive to clone.

This requirement has a bad code smell; it could be that the caller would be fine using a more efficient Rc instead, or that it does not intend to reuse these values and therefore has no need for such a wrapping.

It may be preferable to refactor so as to avoid this requirement and provide the caller more flexibility.