HUGR Model and binary serialisation format.

[ss2165]

Goals:

1. Stop serialising core HUGR structures directly and go via a new Model structure. See `hugr-model` #1326.
2. Switch to a binary serialisation format with efficient table-encoding.
   Unblocks:
3. Text format in/out from model.
4. Stable-ish serialisation.
5. Resolve conceptual issues with "value". See Values and constants #1425.

Tasks:

• Create benchmarks for current serialisation. #1434
• HUGR model structure and conversions from core. See draft: feat: Export the core data structures to a model representation #1414
• Extensions expressed in model structure.
• Implement proto/buffer based serialisation for subset of model and re-run benchmarks. #1453
• Finalise binary format and complete implementation from model.
• Add python bindings to model and use in hugr-py.