Modular HUGRs

[aborgna-q]

We should allow non-ModuleRoot as roots of the HUGR.
This makes them much more composable, and we wouldn't have to define full modules for e.g. replacement graphs.

I think it would mostly incur removing references to the module root;
https://github.com/CQCL-DEV/hugr/blob/e6178ffba838bc6ee22829ed384bfa8086481e27/specification/hugr.md?plain=1#L134-L141

This interacts with #74, as non-local constants still need to go on the global module.