I considered this (and even followed through with it on one draft), but I opted not to do it because right now this version has the nice property that `Name` is only returned from the methods that return graphviz identifiers, I think.

I was tempted to name this type `Id` but we already have the `NodeId` type floating around all over compiler (my primary use case for this library) and I for some reason thought saying `Name` here would be clearer (yes yes there is `ast::Name` in the compiler as well but that is not as wide-spread; in particular I don't think I needed to talk about `ast::Name` in the control-flow graph code).