use std::shared_ptrs pervasively

This allows us to drop all the destructors and simply let reference counting do
its job. I don't think we should take this approach in the checker because
shared_ptrs aren't free, but the generator is not performance critical. Note
we retain a raw pointer for ExprID::type_of. It's difficult to avoid this
without introducing circular references or memory leaks. I think it's fine for
now.