Implement lazy sets

[marcelocantos]

frozen.LazySet should be implemented over the top of frozen.Set. Considerations:

1. Consider whether frozen.LazyMap is warranted. frozen.Map doesn't have many algebraic operations, at least not yet.
2. Incremental caching: As the LazySet materialises, cache all values seen so far in a Set.
3. Allow generative, quasi-infinite lazy sets such as AllUints(). These could be bounded by filters: AllUints().Where("$ < 10").
4. LazySet might need more fine-grained semantics for better optimisations. One example comes to mind: Instead of just IsEmpty() and Count(), it might be useful to have CountUpTo(n int), which will return the count up to a maximum of n, where a return value of n implies Count() ≥ n. This avoids problems with infinite lazy sets.