Garbage collection

[SHoltzen]

Currently all nodes that are allocated by the bottom-up managers are never deallocated. This issue establishes a garbage collection interface for collecting unused nodes. It adds the following method to the BottomUpBuilder trait:

pub trait BottomUpBuilder<'a, Ptr> {
    ...
    /// garbage collects all nodes in the builder.
    /// only the roots provided in the `roots` argument are guaranteed to be preserved. 
    /// Roots may be moved during garbage collection; these new roots are returned in the 
    /// same order in which they were provided.
    fn collect(&'a self, roots: Vec<Ptr>) -> Vec<Ptr>
}

This issue involves scoping and implementing the garbage collecting strategy. Design considerations:

• Ideally, we should not simply garbage collect all nodes that are not referenced. It's often the case that nodes that are used once are often used again.
• Ideally, we should try to preserve as much of the apply cache as possible. We should study what cudd does and see how they manage to do it.
• This API needs to be able to handle lifetimes. All references to pointers should become invalidated after collection (since they can all be moved). Ideally this is maintained at the type signature level. The above API does not do this; we will need to experiment and think about this. Hopefully it does not require adding a trait-level lifetime to the Ptr trait, but it might.