Add support for negative costs

[saulshanabrook]

In #763 it was clarified that our built in extractor should support negative costs, but currently we don't allow this.

egraphs-good/egglog-python#391 was raised in the Python repo by someone who wanted this. They could do this by adding a custom cost function currently, but this has the downside of not working with the builtin set-cost and :cost.

I imagine it shouldn't be too hard to add this? If the extractor in fact does already support it, it would just be about changing the type from an unsigned integer to a signed integer.