hugr-py cannot load CFG edges json-serialized from Rust

[acl-cqc]

Rust produces edges something like:

"edges":[[[1,null],[4,null]],[[1,null],[3,null]],[[3,null],[2,null]],[[3,null],[4,null]],[[4,null],[2,null]],

...(where nodes 1-4 are DataflowBlocks - note the two [1, null] outports for successors 0 and 1 of the same block)

Whereas hugr-py produces

"edges":[[[1,0],[5,0]],[[1,1],[10,0]],[[5,0],[4,0]],[[10,0],[4,0]]],

...(eliding non-block edges)

In practice, both roundtrip, and Rust is able to load hugr-py output, but not vice versa.

[acl-cqc]

This diff "fixes" the Rust by making it output the same as hugr-py:

--- a/hugr-core/src/hugr/serialize.rs
+++ b/hugr-core/src/hugr/serialize.rs
@@ -183,7 +183,13 @@ impl TryFrom<&Hugr> for SerHugrLatest {
             let op = hugr.get_optype(node);
             let is_value_port = offset < op.value_port_count(dir);
             let is_static_input = op.static_port(dir).map_or(false, |p| p.index() == offset);
-            let offset = (is_value_port || is_static_input).then_some(offset as u16);
+            let offset = (is_value_port
+                || is_static_input
+                || matches!(
+                    op.port_kind(crate::Port::new(dir, offset)),
+                    Some(crate::types::EdgeKind::ControlFlow)
+                ))
+            .then_some(offset as u16);
             (node_rekey[&node], offset)
         };

although I wonder if we could avoid is_value_port and is_static_input (misnamed, should be is-static-port) altogether as these seem to duplicate logic in OpType::port_kind - maybe just something like matches!(op.port_kind(...), EdgeKind::Value(_) | EdgeKind::Static | EdgeKind::ControlFlow)