Map from EGraph<L, _> to EGraph<L2, _>

[sgpthomas]

I want to use the babble library with an e-graph that I own. babble expects an EGraph over the AstNode<Op> (which implements egg::Language) type. I have an EGraph over my own language type, let's call it Lang, and it's trivial to write a function from Lang -> AstNode<Op>. However, as far as I can tell, there is not a good way to lift this into a function EGraph<Lang, _> -> EGraph<AstNode, _>`. Am I missing something in the library, or do you have a recommendation for how to do this?

[mwillsey]

Closed by #306