Can `Var` have an `id` field?

For code rewriting, it can be really useful to have more to distinguish a variable than just its name. For example, we might want to do things like "rename all occurrences of this `x` to `y`" or even "convert this AST to SSA style".

This could be very easy if JuliaVariables could be extended so `Var` has an additional `id` field. Do you have how difficult that would be to add? If it would be a major rewrite I could try to find other ways of getting to this.