Support serializing with custom cost model

[saulshanabrook]

Currently the extract in Rust can be passed a custom cost model to use for extration.

The serialization also could be passed the same type of cost model to facilitate serializing using the same cost model that would be used for extraction.

[saulshanabrook]

Sure! The extraction logic takes in a cost model:

egglog/src/extract.rs

Lines 143 to 146 in 529f428

	pub fn compute_costs_from_rootsorts(
	rootsorts: Option<Vec<ArcSort>>,
	egraph: &EGraph,
	cost_model: impl CostModel<C> + 'static,

The serialization does not:

https://github.com/egraphs-good/egglog/blob/529f428d2443f126cd23d1012d7dba980baeed33/src/serialize.rs#L125C37-L125C52

To serialize a cost, each node's cost must be emitted separately and not include the costs of the children.

So you wouldn't want to use the fold method of the CostModel. You would want to use the enode_cost and base_value_cost. To use the container_cost, we could pass the the identity cost for all children instead of using their real values. It would be better to maybe provide in the children costs and then subtract them off the result, or change the interface to allow creating a container cost without it's children, but both of those changes would require changes to the CostModel trait, which might be better to avoid to start.