Module wrapping e-graphs to use custom e-class-ids

[alt-romes]

Idea

It is arguably useful to be able to use e-class ids other than ClassId, so we could provide a module that wraps an e-graph and provides an interface to the e-graph in which e-class-ids are some user-defined type that supports lookups (e.g. instances Ord)

• If we want to associate some data with an e-class, but don't want it to live in the e-class data because it might be used very often, or inherently related to the Id rather than the e-class
• The user-type for Ids might allow discovering equalities on its own before the e-graph has to, since the Eq instance of the user type might take into consideration more data rather than just being an Int.
• TODO

This is mostly motivated by the use of e-graphs in GHC's pattern match checker, where it is potentially useful to use Ids as e-class ids.

Alternatives

We could also say we won't do this as it is a bit ad-hoc and adds complexity, and that

• users can have a datatype wrap ClassId with more data they require, and then still use e-class ids and/or
• users can instead implement this layer of indirection themselves