Roots that are not Modules

[zrho]

@aborgna-q pointed out to me that there are Hugrs whose roots are not Modules. That is currently not accounted for in the model. How do we want to deal with this? There's an argument that the root should always be a module: say it was a dataflow graph instead. The dfg then may refer to a custom type in its signature. But then we'd want a place where we can declare this custom type. If declarations are scoped in the natural way, the declaration can not be inside of the dfg, but if the dfg is the root it also can't be outside.

[aborgna-q]

See #1642 for a proposal solution, making all hugrs define a module.