Allow loading and saving intermediate state for VhlSwarm

[tangentstorm]

Right now, this is configured by the definition of BDDBase in rs:

pub type BDDBase = BddBase<S,SimpleBddWorker<S>>;

I want to say this instead:

pub type BDDBase = BddBase<S,BddSwarm<S>>;

... But that is prevented because the two BddSwarm needs a custom Deserialize implementation, because it has fields that can't be deserialized (the inter-thread communication channels), and these also can't be easily ignored with #[serde(skip)] because they don't have reasonable Default values. Possibly these could be wrapped in Option, but I think it's probably better to refactor so that all WIP information is separated into its own struct.

[tangentstorm]

After #7 (enable swarming by default), this typechecks and runs, but so far, I only implemented serialization / deserialization for the completed work. That's fine for now, because BddSwarm never returns an unfinished query, but I'm leaving this open because soon we'll WIPBase<T>, where nodes don't need to be fully evaluated, and I'll want to capture much more of the intermediate state.

[tangentstorm]

#28 solves the main problem here: we can snapshot after each step in a sub/swap solve... it doesn't actually save the WIP, so for really long steps, an interruption could still lose a lot of work, but maybe saving WIP is no longer important?