Failing hugr decoder with custom operations.

[aborgna-q]

The following test fails while computing the signature of the opaque operation.
(Called unwrap on a None at src/ops/custom.rs:87:54).

const V: &str = r#"{
    "version":"v0",
    "nodes": [
      {"parent":0,"input_extensions":null,"op":"DFG","signature":{"input":[{"t":"Q"}],"output":[{"t":"Q"}],"extension_reqs":[]}},
      {"parent":0,"input_extensions":null,"op":"Input","types":[{"t":"Q"}]},
      {"parent":0,"input_extensions":null,"op":"Output","types":[{"t":"Q"}]},
      {"parent":0,"input_extensions":null,"op":"LeafOp","lop":"CustomOp","extension":"TKET1","op_name":"TKET1 Json Op","description":"An opaque TKET1 operation.","args":[{"tya":"Opaque","arg":{"typ":{"extension":"TKET1","id":"TKET1 Json Payload","args":[],"bound":"E"},"value":{"op":{"type":"V","signature":["Q"]},"num_qubits":1,"num_bits":0,"param_inputs":[],"num_params":0}}}],
        "signature":null
      }
    ],
    "edges":[[[1,0],[3,0]],[[3,0],[2,0]]],
    "metadata":[{"TKET1_JSON.bit_registers":[],"TKET1_JSON.implicit_permutation":[[["q",[0]],["q",[0]]]],"TKET1_JSON.phase":"0.0","TKET1_JSON.qubit_registers":[["q",[0]]],"name":null},null,null,null]
}"#;

fn main() {
    let _circ: hugr::Hugr = serde_json::from_str(V).unwrap();
}

Notice that the signature of the opaque op is not included.

The HUGR was generated by TKET2 from the following qasm file and serde_json-encoded (the same error happens with any other non-supported opaque operation).

OPENQASM 2.0;
include "qelib1.inc";

qreg q[1];
v q[0];

The example is in the branch deser-opaqueop. Just cargo run --example deser_opaque`

[aborgna-q]

TKET1 Json Op signatures are computed based on the operation parameters.
The resulting signature for the OpaqueOp should always be encoded in the json, so that we don't require access to the extension at decoding time.