Const Values are not validated properly.

[doug-q]

guppy emits a const node:

    {
      "parent": 40,
      "input_extensions": null,
      "op": "Const",
      "v": {
        "v": "Sum",
        "tag": 1,
        "typ": {
          "t": "Sum",
          "s": "Unit",
          "size": 2
        },
        "vs": [
          {
            "v": "Tuple",
            "vs": []
          }
        ]
      }
    },

This is invalid because unit types should have an empty "vs".

hugr-cli loads and validates this without complaining.

Calling validate on a hugr should fail if values do not match the types in const nodes.
When fixing this it may be worthwhile to remove the Value::Tuple constructor which can be subsumed by Value::Sum