Move != to function instead of method

[saulshanabrook]

This PR changes how you emit the != egglog function. Previously, you would use the __ne__/x !=y method to create it; with this PR, it is moved to a standalone function ne(x).to(y).

This allows custom classes to override __ne__ to support custom expressions, like how, on arrays, it would return an array of booleans. It also unifies it with the eq function, which behaves similarly.

This is a breaking change, and I could have instead preserved the __ne__ behavior on existing classes and allowed a custom operator on override. This would have been backward compatible, but I thought it was slightly more subtle. Also, for classes that have a custom __ne__, they might also want to emit the builtin != function, so we would need another way of creating that as well.

Happy for any feedback on this approach!