Unify lattices in C++ under a single template

[jhellerstein]

This has proven difficult/impossible. As a result, adding Anna lattice types requires recompilation, and potentially code changes.

[vsreekanti]

The gist of the problem is that our KVS has an abstract superclass, Lattice<T>. There are many subclasses of Lattice, such as BoolLattice, SetLattice<S>, etc. (which are Lattice<bool>, Lattice<std::unordered_set<S>>, and so on). To write a KVS get method that returns a Lattice*, it seems like you can't have a return type without the type template uninstantiated, which functionally means that you need to have a separate type signature for get for every lattice type, which is potentially large because we want to take advantage of lattice composition.

The only solution I can come up with for this is removing the type template and simply relying on class hierarchies for type checking, but we lose typechecking of compositions.