Serialise circuit entrypoints

[aborgna-q]

#370 added a Circuit struct with a pointer to the entry point of the circuit in the referenced hugr.
This struct cannot use the hugr serialization format directly, since it needs to store the entry-point pointer somewhere.

My proposal for implementing Serialize/Deserialize is to store the information in the module's metadata before using hugr's serialization;

• If the root is a DFG, don't do anything
• If the root is a module, add a tket2::entrypoint metadata entry pointing to the right children.

If the circuit entrypoint is deeply nested inside another DFG, return an error.

When deserialising, check for the metadata value or default to the root (if valid).