Allow collections of collections

[Calvin-L]

Cozy does not allow collections containing collections, such as Set<Set<Int>>; It will never generate such a type during synthesis and it complains loudly if such a type exists in a specification. There is no need for this restriction and we should lift it.

This enhancement blocks #100, and must be addressed before we can implement a proper fix.

There are two possible ways to fix this problem:

• rewrite specifications that contain collections-of-collections into specifications that do not contain them (leaving synthesis unchanged) OR
• allow collections-of-collections everywhere, including during synthesis.

I personally favor the first option since banning collections-of-collections helps prune the search space a lot.